I'm fine with it as long as the people asking to sit in are respectful. Like if they ASK and not DEMAND to sit in. Had lots of those, "Hey, I wanna play a song with you guys, you are gonna love it!" ummm, wait until the next set and we'll talk about it. Hopefully he leaves before the break.
I will usually let just about anyone sit in as long as it is not a big paying gig, but if the venue owner/operator/manager/person writing the check asks to let someone sit in, sure I am all for it.
Just the other day, we were playing a tourist area outdoor patio gig, and a nice lady pointed to her son, (looked about 18 years old) and said, "He's a bass player!" we finished the song, and I just handed him my bass, said, "Hey, you wanna play a bit?" OMG the look on his face was wonderful. I stood to the side, and watched as they played Pipeline, and he was noodling all over the song, not really playing the part, but it was fine for 3 minutes. After they finished, he and his mother had the biggest grins on their faces you ever saw! They were so happy, the kid tells me he is from Washington state, never been to California before in his life, and this just made his day!
several people threw down some bills into the tip jar after that, telling me how nice it was of us to let the kid play. I just figure, hey, back in the day, some nice folks gave me that same chance, to just jam for a few minutes. I still remember how it felt, to jam with older cats who had been around the block, when I was just a teenager.
Of course there have been train wrecks with drunk people, but I just laugh it off and play another song, everything is fine. this is supposed to be fun, remember ?