I play what I 'hear' in my heart. Knowledge of theory comes in handy when I want to describe that to someone else. If I can't sit down with someone and show then a lick or a chord progression, then I need to have a written language that will convey it to them in terms we both understand.

As for the scales, keys, and such. I'll admit the only time I dip into that is when I'm stuck, and I need some pointers on what chord(s) will fit a progression I'm working with. I generally 'hear' a song before I try to write it down, and I'm basically learning to play it by ear, just as if I picked it off the radio.