Have to say, I've NEVER done a gig where I've NEEDED a 5string....it may well be a preference for whatever reason (odd keys, a low note or two), but its never been a deal breaker as to whether I did the gig or not.
I have a great 5 string, I don't feel the need to take it to particular gigs, I just use it when i feel I want too. I don't agree that its the standard in "most music"....guys like Pino, Marcus Miller, Wooten, Jaco, Duck Dunn, Hub Hubbard etc etc are all sidemen who do big big tours etc, probably have 5's, but 4's are their go-to instruments.
Si