If you're playing rock n' roll, very basic theory goes a long way, IMO. It's true that some guys get by amazingly with nothing at all, but come on, there are some basics I think everybody should know--especially as a songwriter. For example, little things like the shapes for major and minor scales, knowing what a "relative minor" is, etc. Simple pimple, can be learned in half an hour. In fact, this stuff is so basic and fundamental, I think even a lot of the staunchly anti-theory guys pick it up by accident.
I definitely agree with the "communication" thing. This bass player I occasionally jam with has good feel to his playing, decent fretboard dexterity, etc. but if I start playing a new tune, he's completely lost. I'll say what the chords or notes are for one part, and he has no idea where to find them. I literally have to walk over to where he's standing and let him watch my hand up close while I play the part, or point to the frets for him. Very annoying and inefficient. When the song changes to a different part, it's laughable watching him "hunt" around looking for the right notes--if he had any grasp of basic theory, he'd *know* where to start looking. You can argue that the latter only takes the right "ear", which may be enough, but to me, there's no question that when learning new music, an ear combined with knowledge will be more powerful than an ear alone.