They're a great addition and selling point but just always seem to have some sort of problems. The gigs too far, my boyfriend/fiance/ex/husband, I'm tired from work, my voice isn't right tonight, etc. etc.

I've done duos and trios with four different ones and it was always something. I did all the booking, brought and set up all the equipment including her mic, picked and learned the songs. Couldn't of made it easier for them. One I had could NEVER be on time . We worked a restaurant around the corner from her house and she was 1/2 hr. late the first night!! Did a Wedding and here she comes walking up the side of the room as I'm announcing the Bridal Party !! Her boyfriend was a full time musician with two wedding bands and a big band. She wouldn't work with him because he worked too much. I asked for his advice and he said "fine her for every minute she's late" HA!
Be careful and good luck.