Yeah, the logic and the trends could tend to offset each other, depending on the [logical/legal/moral/personal preference for a buzz] buckets that people fall into.
But if we start with the assertion that pot is no more dangerous than alcohol, then an increase in the number of (legal) pot users is not a net negative. It is merely a shifting of users from one legal drug (booze) to another (pot).
Many people don't like booze, or want more than booze, but can't do pot legally, so they're forced to do drugs illegally. From a legal persepctive, you're already just as screwed if you're doing pot or doing coke/heroin/meth, so there is no real motivation to "just" do pot once you've crossed the legal/illegal barrier.
However, were pot legal, people would not have to cross over the legal barrier in order to do pot, and this would make it easier for people to do exclusively legal drugs as opposed to ever being forced to break the law in the first place.
I know it is all very complicated and involves a lot of perpetual loop sort of logic ... which is why I think we should just go ahead and try legalizing pot and see how it plays out in reality. That's the only true way to find out what happens.
