Mark Kleiman has his response to the Cato Unbound series with: Drug Policy in Principle, and in Practice. A couple of excellent comments have already been expressed in the placeholder post below. But I wanted to take a chance to look at his response a little more in depth.
As this moment (Kleiman’s response) was approaching, I found myself wondering how he would find a way to subtly slam legalizers for no apparent reason. This time it was in the opening paragraph.

Cato Unbound is to be commended for having assembled a symposium free both of the usual drug war rant and of the usual ‹drug policy reformŠ rant.

Note first the use of the “scare quotes” around “drug policy reform.” This is Kleiman’s way of saying that those of us who claim to be drug policy reformers are not actually policy reformers, but are legalizers, which in his view is not… reform. We are apparently not serious folks, but rather fringe elements who think that “crack should be sold at the 7-11.Š (Ah, the ever-present straw man)
How many times is that offensive approach used by prohibitionists? That willful effort to misrepresent opponents’ arguments. If McCain had done something even similar regarding Obama’s views, Kleiman would be up in arms asking “Just how stupid does the McCain-Palin campaign think I am?” Well, then, why shouldn’t we be asking “Just how stupid do Kleiman and Caulkins think we are?”
Kleiman doesn’t go as far out of bounds as Caulkins’ offensive straw man: (“Denying or denigrating an individual‰s right to choose temperance is an extreme position not worth engaging.”) Caulkins, in fact, didn’t seem engaged at all in actually participating in the discussion in any legitimate way.
Kleiman seems to excuse Caulkins’ behavior as:

It seems to me that to some extent Jon Caulkins on the one hand and the Erowids and Jacob Sullum on the other are talking past one another. […]
Jonathan and the others are feeling different parts of the elephant. No surprise that they offer different reports about its nature.

Yes, except that the Erowids and Sullum are talking about the nature of an elephant, while Jonathan is telling us that it’s actually a coconut.
Kleiman really seems to want to re-define the argument to allow better treatment of his favorite drugs, and more demonization of the one in particular that always seems to make him lose all reason — cocaine. Certainly there’s an important place for discussing the specific differences in each drug regarding use, abuse, regulation, harm reduction, effectiveness of education, etc. But I think Kleiman’s thesis is, in fact, a diversion from the intent of the lead essay, not a clarification of it.
Here’s where it really gets interesting. Kleiman is willing to sign on to Caulkin’s balancing of the good and evil of drugs:

Still, I think Caulkins could make a plausible case that the decision to start to use alcohol or tobacco or cocaine or heroin or methamphetamine (in other than pill form) is an ex ante bad decision, because the relatively modest gain from successfully controlled use, multiplied by the probability of achieving controlled use, is outweighed by the very heavy losses from falling into even relatively transient abuse and the extreme losses from falling into chronic abuse, multiplied by those probabilities. The expected value of the gamble may well be negative, even if most people who take the gamble come out somewhat ahead of the game, because the average loser loses more than the average winner gains.

If he stopped here, that would be fine. That is, in fact, a fine discussion to have regarding the free choice of the individual. To weigh the potential upsides and potential downsides, assess the risks soberly (with proper evidence-based information), and make a decision.
But he doesn’t stop there. He moves right on to:

Thus Caulkins has a reasonable argument that voters might reasonably decide to protect their fellow citizens from the risk of falling into substance abuse disorder, even at the expense of missing the pleasures of moderate use.

And he has moved right down the path to supporting prohibition.
But you see, once you add prohibition, the equation changes dramatically. No longer is it a balanced choice. What Kleiman fails to address, pretty much every time he talks about drug policy, is that he is assuming that prohibition works and that prohibition is essentially free.
Sure, he pays some lip service to the costs of prohibition, but never really factors those costs into the equation. To him, there is no reason to consider the costs of prohibition vs. no prohibition, because the lack of prohibition is simply not an option in his mind. He and Caulkins won’t even discuss it.
Therefore, prohibition is a constant, and merely nibbling around the edges of its costs is then a “good thing,” and you never have to justify the total cost in dollars, lives, morality, or anything else.

Caulkins and I could probably go on at length about all the ways in which the costs of the current prohibitions, especially in the forms of violence, incarceration, and infectious disease, could be reduced without allowing the massive increases in abuse levels that would surely result from commercialization. [Boyum and Reuter 2005; MacCoun and Reuter 2001] To offer three specifics: We could reduce violence and drunken driving by raising alcohol taxes [Cook 2007, Cook forthcoming], we could shrink the illicit drug markets and reduce recidivism by using drug testing and swift, automatic, and mild sanctions to force probationers to stop using expensive illicit drugs [Kleiman 1998, Hawken and Kleiman 2007, Schoofs 2008], and we could break up street drug markets, thus protecting neighborhoods, with low-arrest drug crackdowns [Kennedy 2008, Schoofs 2008].

I’ve read most of these, and they are bandaids on the cancer of prohibition. If you’ve already decided that anything other than prohibition is not an allowed topic of discussion, then many of them are not bad ideas.
But what kind of messed up policy discussion eliminates options?
Sure, I suppose it’s nice to be able to discuss drug policy without having to factor in the ravages of prohibition or consider alternatives. But it’s dishonest and unproductive.
Even though the United State has legal and voluntary restrictions on the advertising of alcohol, and even though we have, through education, regulation, and public opinion dramatically reduced the abuse of legal tobacco, Mark Kleiman “recoil[s] in horror” at the thought of how American business would market cocaine, as though we’re powerless to do anything about it — that it’s so completely impossible to even imagine regulations that he has no choice but to “recoil in horror toward the current state of affairs.”
That would be the current state of affairs that has people dying right and left from drug war violence, that has enabled huge black market profits, and spawned massive corruption, dysfunctional foreign policy, and incredible damage to rights, families, cities, the integrity of law enforcement and government (the list goes on and on).
I’m sorry, but the current state of affairs is what makes me recoil in horror. And I have enough imagination to believe that we can come up with a way to legalize and regulate drugs without being required to have a Joe Nose Camel poster over a display of shrink-wrapped Kiddie Krack on the 7-11 checkout counter.
Oh, and yes, this is a drug war rant (or perhaps even a “drug policy reform” rant).
But as Ethan said in comments below:

It’s pretty amazing how narrow the debate about drug policy is, even in a forum like Cato. I rarely–if ever–hear about the huge percentage of non violent offenders in federal and state prisons; the fact that violent offenders are literally “crowded out” of prisons because of the vast number of non violent offenders in these facilities; the politicized science that drives policy making on drugs (ie, Ricaurte’s research on Ecstasy); the increasing costs of the drug war in Mexico (ie, widespread corruption and a fairly staggering murder epidemic down there now); the steadily declining costs of drugs like cocaine on the streets (when our drug policy is supposed to make these drugs more expensive, not less so) the decimation of African American families thanks to a system that imprisons an astonishing one in nine African American men between the ages of 20 and 34. I could go on and on but I’ve gotta say that I’ve been really disheartened by this discussion; reminds me of the “debate” over the Iraq war (where hawkish Dems like Ken Pollack get tons of space on op-ed pages and we never hear from the likes of Scott Ritter.)

Update: Some may wonder why I tend to be so harsh towards the Mark Kleimans and Jonathan Caulkins of the world. After all, they’re no John Walters or Mark Souder. It’s because they’re not. Because they’re academics and they’re smart and I expect more from them. Because I also work in academia and I hate to see talent wasted due to self-imposed limited thinking.
Further Update: Scott Morgan has more

Jane Bussey of McClatchy Newspapers generously demonstrates her own cognitive disfunction (and perhaps that of portions of the Mexican population) in Mexico’s Calderon takes popular and dangerous stand against cartels
With all the violence and death that has occurred since Calderon’s crack-down, the question is how people can possibly support it. Here’s how:

Mexicans have lost faith in many of their institutions, recent polls show, but not in President Felipe Calderon, who boosted his shaky presidency by launching a military offensive against drug traffickers.
The results have been mixed Ö violence is on the rise with the nearly 2,700 killings in eight months, equal to all of the violent deaths in 2007. News of 10 people or more slaughtered in single incident is not unusual.

Results have been mixed? Nowhere in the article does Bussey indicate any actual positive result (other than some seizures and extraditions, which are not shown to have had any effect). The only result seems to be slaughtering. Apparently launching a military offensive within your own country is a good thing, in and of itself, regardless of the results.

“Finally one government Ö the Felipe Calderon government Ö is doing something about it (the violence),” said Victor Lachica , chief executive of Cushman & Wakefield Mexico, a commercial real-estate firm.

And what is that something? Anything beyond simply escalating the violence? Wait, maybe this is it:

Beyond mobilizing some 40,000 troops, who have confiscated drugs, weapons and destroyed illegal marijuana and poppy plants, Calderon has kept a high profile in the war; attending funerals of fallen law-enforcement agents, visiting troops and warning that the war to regain peace will be long, costly and probably mean a considerable loss of life.

Ah yes. He has also attended funerals and told people that there will be lots of death. That’ll certainly put violence in its place.

Calderon has tied his name to a war with no end is in sight. “We are determined to leave the country much more secure,” he said in a Sept. 2 radio interview.

My brain hurts just reading that paragraph.

Washington has embraced Calderon’s anti-drug offensive, …

Of course it does.