Sure there is--when you are operating a multi-ton motor vehicle outside of your home, separated from other vehicles by mere feet, you have no business being under the influence of anything. Or when you are operating on a surgical patient, operating dangerous machinery, etc. I don't care if you want to kill your own brain cells and dope or drink yourself into oblivion--in your own house. But don't you dare bring that lack of perfect sharpness into the lane next to mine or into an operating room or a manufacturing facility.
Of course - but if that's not reason enough to ban alcohol, it's not reason enough to ban pot.