The possibility of someone driving on a substance is not grounds for prohibiting it. And while I am not advocating driving under the influence of anything, the way alcohol warps and distorts balance and spatial awareness is not shared by marijuana or many other drugs. Indeed, judgment could be impaired, and there is a danger of drowsiness with many illicit substances, but the dangers are not any worse than what we face already with alcohol or many prescription and over the counter medications for a wide variety of the substances we're discussing.
ETA: In fact, I would rather have people high on most of the illicit drugs and driving than I would alcohol. Alcohol impairs your spatial awareness and balance as well as judgment, while marijuana may only impair some people's judgment and alertness. Methamphetamine would probably improve many people's ability to drive and reaction time (never mind the sleep depraved insanity :O ) While I am not advocating that people allow others to drive under the influence, if I were to choose what is more dangerous, outside of potent and vividly intense hallucinogens, I would say driving while using alcohol is far more dangerous than any of the illegal substances we're talking about. If you're going to to actually present the situation that this somehow opens a slippery slope, beer has already done this.