If you're given a chord chart, but you only know how to play a Maj/Min scale over it, what kind of lines are you going to put down? If you know ALL your modes, their different positions, their relative minors etc, your lines become even better.  You can't 'learn' perfect pitch, you can learn relative pitch.  If you want to be a musician that only reads dots, then in retrospect, you don't need to know any theory what so ever. If you want to be a session musician who's going to play with Amy Winehouse, then do function gigs at the weekend as well, you need to know your theory inside out to even get a shot at the Amy Winehouse gig.