^^ All great (and sensible) reasons, IMHO, but I see one main sticking point, and that is the lack of a timely field test for impaired driving.(To the best of my knowledge.)
While society has taken great steps towards reducing the incidence of drunk driving, I wonder if we'll be starting again with education around stoned driving - especially with a generation who've grown up with the mantra that 'pot is less dangerous than alcohol, mannn' ringing in their ears..