I never bring a backup and here's why: I'm willing to let someone borrow mine if they need to. If a band on your bill has a major equipment malfunction and none of the other bands are willing to help them out, then those guys are a-holes. You are part of a community of musicians and you should act like it. No its not ideal to have to borrow someone else's gear and you should never ever mooch, but an emergency's an emergency. If they won't do the same for you, don't ever play a show with them again.
That talk of "community of musicians" is lovely.
You do get a bit of that, no doubt, but you also seem to have a significant number of "a-holes" in the "musician community" who are not respectful of other people's property.
As a result of that, I no longer lend anybody anything, unless it is somebody I know personally and I know is trustworthy. I don't care about accidents... but if something happens because someone could not be bothered to have a minimum of care it pisses me off.
Even then, friends sometimes let you down: a guitarist friend of mine, really good guy but a bit... erm, forgetful... with a very unreliable guitar, was getting flak in his band for the state of his guitar on two successive gigs. The third gig arrives and... has he done anything about his guitar? has he f**k. So he asks if he could borrow one of mine. I said yes. I even took the guitar to the gig myself, but I told him I was going to go off later and that he should just take care of it and return it next weekend.
He forgot the guitar at the venue!!!

Fortunately, another friend of mine knew it was my guitar, saw it, and she took it with her then called me.
I can take **** like that from a friend (oh, and he took a mouthful afterwards, rest assured!

) butnot from a random member of the "community of musicians".
I only let my amp be used by other bands if it has been arranged like that beforehand, not because someone could not be bothered to figure out the basic logistics of a gig. Paraphrasing somebody else here earlier... "I am not their mum"
I could step in to help out someone should a genuine accident happen, of course, I do have a heart... but it will not be an automatic "yes" from me. If I have the feeling the guy's trouble is his own fault/mindlessness... then I'm not going to let him experiment with mine. Especially not before I play.