There is no question that the combination of marijuana and alcohol produces extremely impaired drivers. However, the evidence says that people who have used marijuana not combined with alcohol drive just as safely as people who are not under the influence of anything.
As for "No one should operate a vehicle while under the influence of any mind-altering substance", what about caffeine? For that matter, what about benzedrine? The US Air Force issues "go pills" that are primarily benzedrine to fighter pilots who are being sent on long missions. If it's good enough for a fighter pilot, it seems to me that it should be good enough for car drivers.