1) The majority of people incarcerated right now are non-violent drug offenders.
2) legalizing it will a) save tax payer dollars housing a grown man who made the mistake of smoking/buying/ dealing a little weed b) create revenue through taxation and c) partially eliminate a great imbalance in our justice system where poor people (usually your minorities-but not always) get harsher penalties for the same "crime" as their more affluent counterparts. So in essence, it evens things out on that level.
3) part of the revenue raised/ saved can be used for rehabilitation programs or preventative programs that are far more effective in keeping people productive than incarceration.
4) free up MUCH NEEDED law enforcement for important things like oh...I don't know...rape crisis, child abuse, murder, homeland security.....
I am not a drunk and drinking is legal. I am not addicted to porn, gambling or anything else because it is legal. Smoking has been illegal for teens for years now and that hasn't stopped them from doing it all, yet we do not jail our youth for doing it.
Making things legal takes away the glamour, takes away the criminal element and viaolence (when was the last time a man shot another man for simply selling a similar item in the store next door??), introduces the legitimate business and regulates product, condition and safety/health standards that were once not there before.