It depends on the situation, I guess.
I had a female singer that brought her husband to the audition and first rehearsal. Considering she was going somewhere she hadn't been before with a bunch of guys she hadn't met before, I didn't blame her. I'd probably do the same.
If you're doing rehearsal seriously, I don't see why anyone would want to come. For my band, we don't rehearse that often, so it's not like the band is taking much time from the significant other. When we do, it's only 2-3 hours and we work pretty seriously. If there's a bystander, we're not going to be paying much attention. Usually when we've had a bystander, they would watch us for a while and then find something else to do. It's usually pretty boring for the bystander, especially if they've seen the show a few times already. It's a bunch of folks talking about a subject that they may not know much about, using lingo they don't understand, talking seriously about things that may seem trivial to them, playing music that's going to be messed up, hearing parts repeated over and over, stopping in the middle of a tune to talk over a part, hearing folks argue, etc..
In short, as long as they don't get bent out of shape for being ignored for the length of the rehearsal, no problem.