I'm usually OK with someone else using my gear, as long as a I know who it is beforehand. I've been in situations where it's been impractical to bring two rigs, and have used someone's else's rig. I think if the other player(s) is experienced, he or she will treat the gear with repect. Saying that, if it was a few bands, I might be a bit wary.
We're doing a gig with another band soon and I'm using their rig, as we're the opening act. It's an Ampeg SVT AND 8 X 10"