Murry and gargoyle by all means apply your over simplistic Puritanism to your self, but don't assume that because you have not learned to use alcohol constructively that others have not. Does alcohol improve or impair performance? Well that depends. When you learn a tune the state of your mind at the time is also part of the memory. So if you learned, or generally play, a tune whilst drinking (say at a session) you will find it easier to play it well when you consume your customary amount of alcohol. Obviously the converse applies so if you generally learn and play sober drinking will make it more difficult to play well. When I first started out I did floor spots at Folk clubs and I got so nervous it nearly made me ill (barf). A couple of pints of good northern ale helped a great deal. I make my living playing Highland pipes at Funerals, Weddings, and Parties etc. Clearly it is inappropriate to drink at funerals, and I generally stay sober for Weddings unless someone forces my hand. I also play in an E-Cheilidh band and I do drink at those gigs. I have never been to pist to play at any booking.
If you don't want to drink at a gig that's fine but were appropriate sensible drinking is not morally reprehensible.