I play in two bands.
Band A is very casual. If I cannot make it, there is a sub. No worries.
Band B is run by an older guy who wants band loyalty and takes it personally if I or other band members have something else going on for a rehearsal or gig. He perceives it as 'disloyalty' and gets mad, saying that musicians cannot be trusted, and then it goes toward him accusing the band members of stealing his gigs and rants on about how tired he is of musicians. He keeps on a really mediocre keyboard player with awful equipment, who will not learn new tunes from a CD, just because he will show up for a rehearsal and a gig. Unfortunately, the keyboard player has bad timing and no sense of style/groove, and is difficult to play music with. Of course he is available! So, that's the leader's take on loyalty. I have tried and tried to explain to him that the reality of musicians today is that we need to be involved in a number of bands, projects, whatever to keep busy. It has nothing to do with loyalty, its more about making a living or keeping busy. I give my best to each and every gig. I also subscribe to the first booked is the gig I take.