Some lively discussion here.

I'll share a couple stories as well...
I have a pretty strict personal policy against letting people use my gear, but I'm also fortunate that the band leader for my main project realizes that we've worked very hard to put our show together, and has a policy of no one on stage who's not a member of the band.
I can only remember one exception to my personal rule: at a multi-artist ticket we were headlining, the bass player for the band before us showed me his bass, with a tuning peg broken in transit. I could see the look of pain and panic in his eyes, and my heart went out to him. I could tell he was desperate, and the look of relief on his face when I said sure was all the thanks I needed. And even though it wasn't a $6000 bass (limited edition MM Stingray 5), he still babied it. Plenty of stage space, no rogue cymbals (lol), no problems.
On flip side, I was sitting in for an opening gig with a band I've subbed regularly for, and the
headliners asked to borrow
our band's backline gear. The drummer and guitarist knew ahead of time and had agreed, but our band leader clearly stated that since I was just subbing and also didn't know about it, I had no obligation. I was a little uncomfortable with the situation and said so. The band leader had some discussion with the venue contact and the main band, and you know what we saw?
Their bass player went right by us and got his bass amp out of the band's trailer. They were borrowing our gear simply because they didn't want to set up and tear down their own. Needless to say, our BL had more (and less diplomatic) words with the venue contact.
@Brad Johnson, I'm a little surprised that the band leader asked you as the sub if "some guy" could sit in on your gear. Seems more than a little presumptuous on his part, but it sounds like you handled the situation quite diplomatically.. I agree with others, anyone asking to sit in most likely doesn't belong there. I've never asked to sit in, but have been asked... big difference!
5sg.