Last night at one of our regular gigs, a bassist that played with the singer and guitarist showed up. Turns out they had not see the guy in 20 years, he had moved to Nashville years ago and has been there ever since. In the middle of the second set, the singer calls the guy to stage to play a few. He walks up to me, sticks his hand out as to take my bass and said, "let me play your bass". I shake my head and say "no", and he's says "I'm a pro in Nashville", apparently thinking that the word "Nashville" is the golden ticket. I stare him dead in the eye, lean in close, smile a bit and respond "No one touches my gear but me!". The look on the guys face when I said "no" was priceless, like he had been told that that his dog died. He stood there for a second and the said "OK" and turn to the singer, took his acoustic and proceeded to do 3 songs.
After the set, the singer asks my why I didn't let him play, that the guy has been a friend for over 20 years, I tell him that the guy is his friend, not mine, and being that you haven't talked to him in 20 years, you really don't know him anymore. Second, I don't loan gear, I don't let anyone touch my gear. End of story. He thinks for a second and says, "I apologize, I should have asked you first". Not what I expected, and tell him no problem.
I know that there have been several threads about people using gear, for those that are planning on sitting in, be smart, bring your own bass.