A very interesting thread!
I've played bass for years, always by ear, in pop and rock bands, and it has been fine. But I have decided that the time has come to stretch myself, so I had my first rehearsal with a jazz band this week, and that was certainly challenging! I can read a bit, but only very slowly (think of 4 year old with Janet and John book ...)
I could follow the charts, but as someone said, to do that well, you need a full understanding of the genre and the chords etc.. I guess I got away with it (up to a point) as they didn't ask me to leave, but it's definately time to start studying!
Dave