I would say that most arguments against legalization are rooted in basic moralistic notions that drugs are "wrong." As I understand the science, studies, and literature, marijuana is not really a "gateway drug", nor is it more "unsafe" "addictive" than alcohol, and it's pretty clear that minors can get their hands on drugs as easier, if not easier, than alcohol.
So, if I were to argue the view that marijuana should remain illegal, it seems the primary arguments must be economic. Here in California, medical marijuana is legal. In my East Bay neighbor of Oakland, the city just raised a tax on the sales made by the handful of marijuana dispensaries that act legally. One of the arguments that was bandied about was that if the government legalizes weed and regulates and taxes it, you then have a situation where there can be a black market to buy untaxed and unregulated weed on the cheap. The comparison, I guess, was to buying moonshine from your local hillbilly rather than the stuff at the liquor store which is highly taxed. Or the fact that people really do drive to North Carolina to buy untaxed cigarettes and then sell them in states like California where the taxes are high. The concern was that if this black market remained, then you still have the risk for a criminal syndicate to grow around that black market, just like the current gang violence which is based in large part around the drug trade.
I would say this is a weak claim and the risks are minor. Also, once a culture of legalized and regulated marijuana is accepted, people are likely to simply buy the taxed stuff rather than their black market dealer--just like the vast majority of people buy their smokes and booze from legitimate retailers, rather than the black market. But at least the logic of this argument is more or less sound.
The other arguments I have heard are about reduced productivity--that people are going to become stoners, and go to work high or not go to work at all, and the ease of access to drugs will increase this. I'm largely unconvinced, but then again I have many friends who are huge stoners and really don't do much with their lives. Of course, I don't think legalization would change their lack of motivation.
Finally, I have heard arguments about the health-care costs of marijuana, and that legalization could increase the incidence of lung or mouth cancer, or things like that. Once again, the increased costs would seem marginal to me, and certainly pale in comparison to cigarettes which remain legal.
So, I'm not sure I actually did what DJ wanted--trying to present the arguments in favor of prohibition--rather than simply discredited additional arguments. Oh well.