most ridiculous statement, how many people have had a beer and go absolutely manic and paranoid to the extent they fear for their life? I work in a psychiatric unit and the pretty much every patient has issues as a result of drugs, in particular cannabis, even sometimes just one smoke is enough to prove that keeping it illegal is the right thing.
There is a definite link between mental health issues, most notably schizophrenia and cannabis use. The evidence isn't strong because very few people just smoke cannabis. E.g they will have smoke cigarettes, done a bit of alcohol etc... so it's very difficult to attribute it to just cannabis use.
Yes people smoke, but of those that smoke I'd say about 10/15% that I know do weed, in no small part because of the fact it is illegal. Society dictates that weed is bad, this in turn means fewer people do it. Legalise weed, those stats will rise, the number of people doing it will increase, maybe even to the level that pretty much everyone that smokes will smoke weed just as much as they do cigarettes. This will in turn mean people with mental health issues will increase as a result.