The only issue is that weed has three times as many carcinogens as normal cigarettes, so if it being legal entices more people to use it, which, if we're all honest with ourselves, could very well happen, and then those people get addicted to it (and yes, you can get psycologically addicted to weed the same way you can get psycologically addicted to anything that you do a lot), and then those people all get cancer, because they are three times more likely to get it, and now the medical system has to treat tons more lung cancer patients, and that is a huuuuuuuuge drain on resources. The only debate is whether that drain is offset by the current drain on prisons or holding cells for holding a bunch of minor possessors for a short time.
The only real way to get rid of the money drain is to shoot drug users on sight, which is a permanent and cheap fix, which also stops new users from trying it because they know how dangerous it is (not that drugs aren't already life or death). Then demand for drugs drop, and because there is no demand, supply will have to drop as well, and the drug empires fall, and then BAM: No drug users, no drug dealers, no drain on resources. But the moon will turn into cheese before Congress passes that motion.
Personally I hate drugs. I view drug users as idiots who don't have an ounce of self control. I know that serious abusers have a serious problem, but they dug themselves into that pit. No one is born addicted to heroin or alcohol. People have to make that choice. And I despise them for it. I have friends, good friends, who have everything going for them right now, one runs track and is one of the fastest runners as a sophmore, but she got kicked off the program for alcohol abuse, and the other is fighting marijuana addiction after seeing her magic 4.0 disappear after she lost her ambition. It took 4 months to convince to her that she had a problem, and now she has turned to alcohol, so I guess it's a little personal, but still: drugs ruin lives. Why make that legal?