I've lent basses and amps to my brothers' old bandmate and friends of theirs in the past when their own had sh#t the bed on the day of the gig because I'm a dead nice guy and all, but I used to hate when other bands on the same bill as us would stroll up and expect to use our gear because they hadn't been arsed to bring their own and we had.
I had no problem sharing (sometimes it's necessary) if it was pre-arranged, and I've used other people's amps plenty of times that way, but they could f##k right off if they just turned up and started messing about with mine without even asking first.
The "don't touch my amp" attitude started after I agreed to let one lad use my amp if he didn't change my controls and used the other channel, sure enough as soon as I turned around he was peeling off the tape and f###ing about with my settings! He saw his derrière when I called him out on it and I couldn't believe someone could be such an enormous cheeky t##t.
Why the hell would you join a gigging band without access to your own guitar or amp?
How do these people rehearse?
So yes share gear if they've asked well in advance or I know them, no if they're cheeky ill-equipped bastards.