All about horses for courses and how sober / professional one has to appear to be. However, I can't be convinced that either I am[i] that[/i] good that having a few pints makes me play noticeably worse, or I'm that bad that being totally sober allows me to play better. However, I will concede that everyone having a few pints makes unforeseen cock ups more likely, again, I'd be surprised if anyone notices and there are tonnes of other factors just a likely to cause such problems.
I don't class having a few pints as being pi$$ed though. Not unless I've not eaten all day or had little sleep. In which case a few pints usually results in some kinda disaster - falling off stage, pulling amps over, forgetting to plug in etc etc.
I understand certain gigs having a non-alcohol policy (I've played a few) and bands being the same (I've never been in one) but I'm of the opinion that this is more of a safeguard against people getting pi$$ed rather than having a few beers over a couple / a few hours.