Couple of good points here. Yes, they need permanent people, and need to learn how to keep them. But subs will always be needed. It just an ego thing, especially with jazz players where there is often a lot of pissing contests going on. " you need a chart?, what are you an amateur?". I don't get this attitude but it is there.
Well, fortunately for me, I suck, er, am very limited on the bass, music-wise. So I will likely never have to deal with this.
Another TBer was mentioning a similar situation with bluegrass musicians. I think anywhere "keeping it real" is a primary motivation image-wise you can run into this. It just makes it more crazy where a high amount of technical work is part of the picture. Not so much a problem in, say, punk or blues, unless lyrics are concerned. Like others have said, orchestras use charts, and therefore use stands. Why shouldn't jazz trios, given the technical demands of the music? Because you can't read charts with sunglasses. Duh.