Don't know about the violence thing, but I was there when the "musicians" took over the one bar out of either pub that wasn't already a musician's session. I say took over because it was not a case of them taking a turn to play a tune in between the singers - because they didn't even have the manners to stop playing during songs that it was obvious that they didn't even know, but effectivly dictated both new melody and timing to the singer. One funny note was the way one cheesed off singer dealt with it, i.e. each time the "musician" started playing along, the singer changed the key in which he was singing, after the third time the "musician" got the hint and stopped trying to take over the song!
But must agree the hallway may have been a pain for the staff but it had good accoustics.
Jude