All a "band" is, is a group of people who happen to be playing music together. It isn't a marriage. There's no rule that if a person plays with one group on Tuesday he can't play with someone different on Thursday. Frankly, I resent when a person I play with becomes possessive of my own time, as though the fact that I play with him gives him the right to dictate what I do when I'm not playing with him. Everyone's a free agent.
Also, I've had the experience on several occasions of turning down good opportunities in order to concentrate on one project... only to have that project disintegrate a couple of weeks later for reasons that have nothing to do with me. Get burned like that two or three times, and I am no longer willing to put all my eggs in one basket.
On the other hand, there is such a thing as keeping your commitments and being honest with yourself about whether you can really spare the time and flexibility to be able to meet the goals a group you're playing with has. If you say you'll play with a group that wants to gig out regularly, but you fill up your calendar with gigs with other groups, you're going to be a problem for that group, and should step away and let someone else do it.