On the other hand, I don't think hanging around encourages the band to find it's own following. If these are original bands, which I imagine they are, one of the band's main jobs MUST be finding people that like the music. There should be advertising, word of mouth, etc. A real effort to get people to shows. Hanging around out of courtesy could give the band a false sense of status. It may make them believe they're getting more people than they are.
If I play first, then two more bands play before the final band, and the entire crowd has dissipated, then that should serve as a wakeup call to that final band. Bands are hard work, they take a lot of effort and business sense to make successful, and things don't just generally fall into people's lap. If I'm that last band and there are no people there, instead of whining at other bands and asking them to stay out of courtesy, I'm gonna make damn sure I find people who WANT to hear the music to be there, whatever it takes.