When I was doing lots of club dates when I was very current in terms of what people were playing, I got called / fell in to lots of these.
Granted, they were generally with bands I probably would not have wanted to work with full time, and usually they were in places somewhat off the beaten track for the big local bands. I was known to be a non-drinker / non-drugger and I always seemed to follow someone who was too indulged to carry on and the band had just had enough.
So, in that case, I was instantly a hero since I showed up on time and had bathed recently, could play in tune and knew all the tunes, and could remember where I parked.
The thing is, you never exactly know how any given band is going to play a given tune. So it was a great education of having to play it right / right now, and I developed a serious set of radar between my ears. You learn how to listen real fast, how to read other players, and how to find your spot in it all right now. And of course if the rest of the band was happening, once they had a bassist that would fit in and go with their flow, it would tighten up really quick.
Of course I'd prefer some preparation and rehearsals or charts of gig tapes, but this sort of thing really built some backbone and helped my playing a lot. I came to really prefer joining an established band, as they were already doing what they do: All I had to do was fit in. Plus I really took some satisfaction that as much crap as I took for not drinking and doping, I'd ultimately have the last laugh.
J o e y