Your argument is not necessarily contrary to mine. PSAs talking about stoned driving are a way to help ensure responsible use, at least as much as drunk driving PSAs do. The fact is, if pot were legalized, you would probably see an initial jump in the number of people using marijuana, at least I would probably initially use it more than I do now, and whether you like it or not marijuana does impair judgment.
Many people that smoke pot don't have a problem with getting in a car and driving stoned because unless they happen to reek of the smell, they feel, and likely can, get away with it if pulled over. That doesn't mean that they are safe to drive because when you're not trying to focus your mind tends to wander greatly from any task at hand, including driving.
My point is that if an officer pulls over a driver for erratic driving, a stoned driver may be able to pull it together and explain away the driving because pot does not give off the telltale signs of intoxication such as alcohol does, but that does not mean the driver is not intoxicated and safe to drive.
And by the way, whomever modded my original post as flamebait, that's just childish.