It's not the drunkeness that matters, it's the impaired judgment. It's the fact that all too often people with impaired judgment perform actions that injurious to others, in ways minor or major, often without even being aware of it.
An extreme example would be the story we saw a while back about the father who flipped out on acid and chewed his infant son's face off. Impairing yourself on acid while you're responsible for the care of a child endangers the child, and as Balog points out, that is a form of harm against the child all on its own.
A minor example would be a punk who's so drunk that he doesn't realize that puking up on your wife is unacceptable behavior. It endangers others to impair yourself on alcohol so heavily that you don't realize you're assaulting someone. Again, it's a harmful act all on its own, without needing any overt damage.
It gets back to the fact that we make
driving while intoxicated a prosecutable crime, and rightfully so. We don't wait for the act of
wrecking while intoxicated before involving the legal system.
I'd also like to point out that the OP was about medical marijuana, not decriminalization or legalization.
And the thread remained about medical mj, right up until we tried to cast alcohol drinkers as hypocrites for not supporting drug legalization. I suppose it was near inevitable that this thread would head off into the legalization vs criminalization debate.
But you're right. I'll quit with the legalization vs criminalization stuff.