I answered no, but that's not always quite true.
A few times in the past, I've had people ask if someone could come along to just watch the game. Most times, I just say no.
However, that's just if someone's coming over to more or less watch the game. On the other hand, if someone is, say...thinking about joining in the game, well, I wouldn't mind them just kind of hanging around for a session. The same with someone who's going to join the game, but doesn't have a character yet so wants to get a feel for where he might fit in.
There are any number of reasons that I might allow someone at the table. But to just watch and with no purpose beyond that? Eh, dun like it much.
In particular for people that don't even play RPG's. I've had one or two people in the past ask if they could bring along their S.O. or something like that to just kind of hang out and presumably not get jealous. Eh. While I'm all for trying to get new people into the game, I want them to at least be somewhat interested to begin with.