Most of it's been said but yeah, it's sh*t when people just turn up expecting to use your stuff and, even worse, just using it without asking.
Another thing that annoys me incredibly, is, after you generously allow someone to use your expensive guitar head, they go and stick a pint on top of it whilst they're playing. I'm no expert, but some people are infuriatingly clueless (and usually talentless).
When I started gigging with my new band, I'll be taking my beat up Laney cab rather than my nice Ashdown ones. The Laney's quite cumbersome, but it's done 100s of gigs and been thrown around all over the shop but is still going strong and sounds fantastic. However, we won't ever be headlining, so it's unlikely I'll be needing it anyway, but it's always good to have something as backup in case of either breakage or there being no cab when you get there.
When I was promoting shows, and usually when arranging shows for my old band, I would always email/call the promoter (or headline band) to sort out backline beforehand. It's not hard, just costs a few minutes of your time.