http://cyber.law.harvard.edu/evidence99/marijuana/Health_1.html
"I must preface these statements with the remark that blah blah blah
The physical harm you refer to all has to do with how it is ingested. That may change drastically with legal markets and lower prices. Higher quality pot means you have burn less plant matter to get the THC. Vaporizers might become more popular. Maybe, people will start eating it. It's hard to know, but I think legalization would reduce the physical harm as people consider that more as a factor in how they use it while considerations for costs become less decisive in the method of ingestion.
As for the mental health effects... What you cite talks about marijuana's effect on people with mental disorders (still poorly defined ones causing problems in diagnosis). Where is there proof that it causes the disorders?
There are lots of legal things that have bad effects on some people. For instance, peanut butter, milk and numerous drugs.
If you have a heart condition don't take Viagra but don't cockblock the rest of us.