This is a good point. To me, the question comes down to whether or not drug use would increase if all drugs were decriminalized/legalized. Honestly, I think it would. I think legalization would give drugs an air of "legitimacy" that would lead some people to think that it must not be that bad if the government has given the green light. I also think it would increase availability and maybe lower price (I'm not sold on that) and more people would get involved with drugs. I think more people using drugs would be bad. That's all my opinion though.
It won't get any better air of legitimacy than drinking or smoking has. Probably less social approval than smoking.
As far as more people getting involved in drugs? It's plenty easy to get involved now. I'm not saying use would go down, but I don't know how much it would really go up, seeing as how easy drugs are to find right now if you want them. I'm sure it would go up a bit, and then level off, and decrease again after the newness wears off and people say "that's it? Really? That's all it does???" But that aside... I think in a free society people have the right to make these decisions for themselves. That's really the bottom line.