Good grief, I'm going to have to quote myself as everyone else seems to be talking horse crap.
That's not addressing the technicality of the argument though, I think you'd be hard pressed to find anyone who would agree with driving whilst impaired for any reason.
The difficulty with something like this is that someone may consume some THC and then get banned from driving a month later, clearly having a puff on a funny cig will not intoxicate you for a month, so a line will need to be drawn somewhere.