Since I play guitar, bass and keys, I get a lot of calls to sub when my own band isn't working. Initially, I took on all I could handle; but now I'm a lot more selective. Some not-very-good bands need a sub because they lost or chased away their regular. I also found that I had to un-learn some songs so that I could play the same wrong chords as the guys I was subbing for in "those" bands. In my own group, we have "regular" subs for bass and drums because our regulars have day jobs that sometimes put them out of town. The subs are good players who know our stuff; but we don't take a gig if the lead singer or a regular sub can't make it.