Something to consider: Marijuana use in the Netherlands, where it's legal, is much lower per capita than marijuana use here.
I'm skeptical of cross-cultural comparisons like this. People use the same sort of argument to claim that since Japan has nearly absolute gun control and a very low muder rate, for example, that we should ban guns. Having lived in Japan, I've seen what an apples and oranges comparison that really is.
By the way, I've repeatedly said that I think a good case can be made to legalize marijuana in a regulated way but that tying marijuana legalization to a philisophical argument to legalize all drugs isn't the way to do it.
I also want to point out that the Netherlands does distinguish between marijuana and hard drugs like heroin and crack.