I'm generally not a fan, but can understand that situations and budget can dictate their use in some circumstances. We've certainly never considered it in our trio.
I'm also not really a fan of the incognito sidemen. To show the way to do that, I saw The Darkness the other night. The shadowy guitarist/mandolinist was dragged into the spotlight several times, hugged, embraced and name-checked. The drummer came out front to sing lead on a song and the drum & bass tech stood in on drums, again with huge acknowledgement. (It was a great gig, and what a great self-effacing frontman Justin is!)