Aloysius

Getting back to the OP it's important to note that knowing music theory and being able to read standard notation are not the same - many guitarists have a good knowledge of theory in the sense of being able to find chord inversions and substitutions and knowing what notes will sound good over a particular harmony, without being able to read standard notation. In fact guitarists have only been using standard notation since about 1770, before then it was always tablature of some sort.

I do about 60 to 80 gigs a year (on guitar) and probably only 3 or 4 of them would require reading - for example if I'm working with a classical singer who gives me a score or working with music theatre. Frownland is right about western notation having a very specific bias for Western sounding music. Many flamenco forms work very badly in standard notation - a solea for example tends to have accents at the end of it's phrases, when bar lines imply accents at the beginning. a siguiriya has a rhythm cycle of 2 + 2 + 3 + 3 + 2, which ends up as a mess if you try and use time signatures. Indian music fares even worse.