A common rhetoric you hear from prohibitionists is, "Don't we have enough problems with alcohol?"
I would argue that we do. And from experience and reason, I would say that allowing responsible use of Cannabis would curb use of other, dangerous drugs.
As a real abstract, simple model, say a slight bump in Cannabis consumption curbed alcohol, tobacco, & prescription drug abuse just 1%:
Since there are no recorded Marijuana-induced deaths, a bump in use would just be multiplying zero. Let's drop the death rate from the others just 1% (Correlating with use. We're keeping it simple.)
Alcohol causes ~100,000 deaths/yr--1,000 saved
Tobacco causes ~400,000 deaths/yr--4,000 saved
Scripts cause ~30,000 deaths/yr--300 saved
There is no hard science here, just an idea to keep in mind. It is reasonable to believe using this simple model that allowing Cannabis consumption would save over 5,000 drug-induced deaths every year. Next time someone claims that we have enough problems with alcohol, claim back that we have too many.