Do you smoke? Have you ever gotten high? Have you ever thought that maybe those drugs can affect your performance in a positive way? It surely does for me. And actually, that is the intent for those prescribed medications, to make things better and most of the time the do make things better. I don't need to be on drugs to have an accident. Accidents happen all the time to completely sober people. For example, women drivers.Pretty much what Guild said, it doesn't matter how it affects you - what matters is that there is a distinct possibility that one time in your life it will affect you differently. Those kinds of rules are made for the benefit of the majority, not the benefit of the smoker, drinker or whatnot. It's scientifically proven that alcohol and drugs, be it prescribed or "recreational" affect your performance, and it doesn't really matter if you personally think that they improve yours - you have to be mindful of other people.
So, unless I have an accident or my pot use negatively affects other people your argument does not pertain to me.