I am firmly against slavery, and so we shouldn't let any drug addicts make us into slaves.
Pot bans also affect non-welfare recipients; your goal can be accomplished with no collateral damage by conditioning welfare on the passing of drug tests - or, for that matter, an end to welfare.
Do that first, and then come talk to me about legalizing that crap. As it stands now, I already have to pay into the kitty for the potheads I know on welfare.