I would much rather have my kids smoke too much marijuana than drink too much alcohol. With pot, the worst that'll happen is they'll be lazy, have a smoker's cough, listen to awful music, and maybe get lung cancer. Stack that up against cirrhosis of the liver, delerium tremens if they can't get a drink, driving like hell, vomiting, acting out their violent impulses...there's no comparison.
As it happens, when I have kids, and they get to an age where they're starting to think about picking up bad habits, I'll teach them moderation, something we're not too good at in America. I hope they'll see the middle ground between "Just Say No, and turn in any friends you see ingesting non-government approved recreational substances," and "Let's get wasted!"