I guess after about 25 years of playing music around here I know most of the other bass players within 100 miles of here, at least all the ones that play out regularly enough that we'd run into each other at gigs. I can only think of a couple that are reckless enough that I wouldn't let them play my Alembic. Honestly, I can't imagine them asking. I can count on the fingers of one hand the times I've ever sat-in, or been asked to. It happens, but it's just a rare, friendly gesture. The last time was a Christmas party gig last year, the other bass player, was a former bass player in the band, and yes, I handed him my Alembic. He was very appreciative. Played and sang one song, beginning of a set, and handed it back. He was understandably blown away, never having played one. I remember that feeling well. For me it was a Distillate 5-string, in Nashville 23 years ago.
I can't really lay any guilt or blame on anyone though... I do repair work on the side (or used to) and I've seen firsthand what a moment of carelessness can do. I shudder to think about having to send one of my basses back to Mica in two pieces, but it could happen to me just as easily as anyone.
For what it's worth, I recently offered my Persuader 5-string to the guy subbing for me in Harwell-Grice Band this Fall/Winter... knowing what that bass means to me, he wouldn't take it.