I have a cousin who smoked marijuana and only marijuana. He became psychotic and is now on medication for the rest of his life. A psychiatric nurse I know has had patients permanantly affected like this from just one joint. It is a horrible drug and it can destroy people and families. A lot of people argue "I smoked marijuana and it didn't damage me." Perhaps not. Perhaps they do have some damage to their brains that they cannot see. I think it's like tobacco. You can smoke your entire life and not become seriously ill from it or you can be one of those who are who will pay the price early. No one can predict how their bodies will react and it's just not worth it.
I see marijuana as being more akin to alcohol. For the majority of people, alcohol is a slightly mood altering substance that they enjoy in moderation. For a minority, alcohol is something that changes them into people they'd really rather not be.
In some cases I know, while it may look like they tried marijuana and then had psychiatric problems, the truth is more along the lines that they were already experiencing troubling symptoms of mental illness, they tried marijuana in an attempt to self medicate, it didn't work and they decompensated from there.
As a society, we've decided that it's okay for people to buy alcohol; we hold anyone who does something wrong under the influence responsible for their own actions. Because the majority of people can handle alcohol, we've decided that the onus is on the minority who cannot to avoid it.
While it may be possible that moderate marijuana use causes some type of subtle brain damage, it must be incredibly subtle because it hasn't been found. Moreover, there are some pretty famous users whose marijuana use did not seem to impair their intellectual achievements (Carl Sagan, for one, was rumoured to be a pothead).
Marijuana does have a couple legitimate uses for which there are no effective substitutes--as an anti-emetic, for instance I know someone undergoing chemotherapy who has a prescription for the pharmaceutical equivalent, marinol. Doesn't do her much good because when a person is highly nauseous, keeping a pill down is more of a challenge than they can handle. When she can get hold of pot to smoke, her post-treatment nausea becomes a minor inconvenience rather than a three day ordeal where her life basically stops.
I find it ironic that the whole campaign against marijuana use started off from the selfish interests of a single company: DuPont. When they came up with the first effective, reasonably priced substitute for hemp rope, the company decided to try to get farming hemp made illegal in order to boost their sales. The whole "reefer madness" campaign was the result.
Since then, the anti-marijuana attitude has run amok. Many billions of dollars have been spent on "education," enforcement and punishment, which could have been spent with much greater effect in other areas.
Still could--think of the backlog of evidence for DNA testing. In some states, the waiting list is as long as 5-6 years (longer than the statute of limitations for many crimes!). If we could take the money being dedicated to controlling marijuana use to upgrade and expand lab facilities, that backlog could be eliminated in a year.
I'm not trying to make light of the tragedy your cousin experienced. It was a tragedy but it's akin to the same tragedy that alcoholics and their families experience. As a nation, we've decided that alcohol should be legal even though there are certain to be people who have a bad reaction to it.