I think more people will smoke it if it was legal than if it was illegal, this might be a public health issue (more cannabis related diseases) but thats pretty much my view on the topic. I wouldn't drink alcohol if it were illegal (prohibition style that is) but it could turn to the point where we have marijuana bars or something and you have a lot more people out there getting baked than that would of normally.
It's open to interpretation whether that is necessarily a bad thing.
(Im on the left wing and i used to follow all the left represented including legalising drugs like marijuana but then i realised i dont actually believe all the left has so i decided to go with an approach of whatever works, no matter what side of the spectrum, lots of people here should do the same i reckon)