Reading, while it is a helpful tool, is not always necessary. You'd just not really get calls for session work where it is a sight reading gig, and there is not a lot of that around much anymore. It is quite easy to get by playing copy dates because you can learn "songs" without learning "music" (they ARE 2 different things). But the statement can never be made that a player who read sis somehow "better" than a player who does not.

The thing about someone with a background comes into play where you go to a rehearsal and the guy leading the song will say "This is in C." You can then immediately envision C-F-G with a Dm, and Am, possibly an Em tossed in somewhere for flavor. THOSE guys are easier to lead around the maze of a brand new song than the ones who have to hear a song 150 times before they know the chord changes. The readers and more heavily steeped in theory players are likely listening to the song for the first time and writing a chord chart, where a guy who doesn't have the chord recognition tool in his bag wouldn't know what to write, only to remember that "at this place change to this chord".

One great example I can give you was a spot in a song in C where there is a breakout section that went Dm, Bdim, F. Gsus, G.... to say that to someone who doesn't know what notes make up that Bdim, you may as well be speaking French to a Cuban. But to tell that guy "B, D F, G# or Ab (however he chooses to think of that note - I would say Ab)" then he can play it. And he has probably played it before without knowing he was playing a Bdim.

It's a personal choice that I don't like to learn by ear, which comes from not liking to play other people's songs like they played them. But back to what I said earlier, and closer to topic, that doesn't mean I think I am better than someone else because they never learned theory. I just have that "tool" in my bag.