I said this earlier in the thread but I'll repeat it. The increase in MH problems are very likely caused by the fact that a lot of weed has now been bred to have a ridiculously high THC content. We are actually somewhat evolved to consume cannabinoids in one form or another (eg eating animals that eat marijuana plants) and benefit from it, but we're definitely not evolved to consume these highly psychoactive strains. Like so many things, the dose makes the poison.
Also, bearing in mind that correlation does not prove causation, it's quite probable that people with existing MH conditions attempt to self-medicate with it.
Prohibition is what causes the majority of the problems that are being pointed out here. A lot of people who use it are buying stuff produced dangerously and potentially exploitatively, using pesticides etc. Making it legal would resolve all of this, and also raise revenue via taxation.
It's got amazingly useful medical properties if used correctly, eg medical marijuana prescriptions and CBD oil.
GW Pharmaceuticals has even developed a spray called SativeX for treatment of MS symptoms, and we'll see more of that over the next few years.
I say all this as someone who personally doesn't like being stoned or care for the smell. But blind hysteria on the issue is unhelpful.
As for it being illegal - well, so was homosexuality at one point. There are all sorts of stupid and often immoral laws still interacted globally. Blindly obeying them is not a good thing.