I have been fortunate enough in the past to have played some multi-band gigs (more than 4 bands) where I've liked most of the bands on, so if I hadn't have been playing, I probably would have been in the audience.
If it were less than 4 bands playing, then you definitely need to stay & support the others. I remember many gigs where we would be driving home in the wee hours because we'd stayed at the venue til all the bands had finished, helped load out etc. and then driven back.
If it were at a small venue, single stage, multi-band affair, then yes, stay, watch & support the other bands. If it were a festival style, multi-stage, multi-band arrangement, there could be some excuse for ducking out early, but you should really show some effort to support the other bands playing "your" stage, or make an effort to see other bands on other stages.