Never saw them live, but I think I'll be the only one screaming along and jumping up and down if they were playing Happening, Bird Dream or some other lost gem from this era. Or something off the first bunch of Frank Black records.
first time i saw the pixies (2004), someone was "singing" along to velouria, right into my ear. it was at that point that i decided it would be a good idea to take one step forward and let the moshpit take me somewhere else.
I think the worst is when I've heard people sing along at Frank's solo performances. The concerts are usually held in smaller venues, so the one singing along really can do a great job annoying a lot of people who paid to hear Frank sing.