what harm is the odd Wild Rover or two going to do

Off the top of my head: once you've done one song they know, some punters may think it's a request show and ask you to do Streets of London, Black Velvet Band etc - or for that matter Angels, Chasing Cars etc; punters may stop paying attention to the stuff they don't know, thinking there'll be one of the good ones along in a minute; and if you go down well, you may get asked back on the understanding you'll do a set full of Streets of London, Chasing Cars etc. Careers have ended on less.