A position you could take, which I find interesting and persuasive, is that the illegality of marijuana, and all drugs for that matter, helps sustain prices for people subsidizing their income.
I wonder how people understand the extent of drug dealing as a second job. Not to party with the proceeds, although not to say that doesn't occur, but as an actual revenue source to pay the rent.
It would also remove counter-culture elements to drug use/abuse. In so far as one recognizes implicit challenges to the political/economic structures of capitalism via drug use, one might be resistant to undermining whatever power might be mineable. That is, don't defuse street-level angst and resistance.
But then that idea ought to be tempered with the realization that such use might reproduce one's economic and social situation (and possibly oppression).
Of course, another suitable argument is whether the government should allow drug abuse on a moral level. That is, do we have a social responsibility to protect citizens from abusing their personal bodies and/or disseminating a nihilistic attitude to others.
hmm, my advisor, Elliot Currie, writes that drug laws wouldn't do much for crime if we reduced penalties. That's a very glib synopsis of his position, but I'm about to run. If you pick up "Crime and Punishment in America" or actually I think "Reckoning" addresses this question head on.
Keep in mind that both he and I believe that drug abuse/crime can be reduced by addressing impoverishment head on. Reducing drug penalties is necessary due to how draconian they currently are, but we both feel that such action in itself would only be treating the symptom rather than the cause. So you could address the underlying aspect of the issue, that decreasing penalties is expected to decrease crime. Would it really? Only in the official record, but not in the area where it really counts--overall. That's my best short answer. If you have any questions especially after you read either of those two books (they are written for popular consumption, BTW), ask me some Q's.
Here's a link to an interview I just googled real quick:
http://www.pbs.org/fmc/interviews/currie.htm