From Milambers website link:
"Smoking pot interferes with one's intellectual performance and impairs thinking, reading comprehension, and verbal and arithmetic problem solving."
The above is why I'd never smoke 'mary-jane'. I saw the effects in others. Being a musician it's part of the culture, but something I've always decided to avoid. I like thinking and feeling motivated.
However I agree with decriminalisation - of all drugs. Get them out from the hands of drug gangs and the underground, and out in the open so the terrible consequences of drug addiction is visable to all, not hidden in arcanely attractive countercultural circles.
Remove the mystery and the 'rebelliousness' and show it for what it is. Dependency on a particular substance to feel o.k. for a little while.