For myself, I've always kinda felt it was bad form to ask to sit in with a band that I don't know well (and frankly I don't usually ask my good friends, either!). With others, I try to deal with this kind of thing on a case-by-case basis. If it's a drunk who comes up while we're playing, no way. If it's a drunk, period, it's no thanks.
If the guy asks me during the break, and is respectful, friendly, seems to know what he's talking about, and it's cool with the rest of the band, I might say yes. I'm less likely to be happy about it if he approaches one of my bandmates about playing MY gear, and am more likely then to say no.
But I reserve the right to say yes whenever I feel like it!