I've been playing guitar for ~5 years (5 years electric, 2 classical), and I'm teaching myself piano. I've been taking guitar lessons for the past 2 years, and I'm lucky enough to get a professional guitar teacher who actually knows/applies theory. In my eyes, anyone who is serious about making music should DEFINITELY know music theory. It's almost inexcusable to not know the basics. There are so many resources out there, so it's easy to find. Now, I'm not trying to seem like an asshole, but most (MOST, not all) songs made without knowing theory are bad. plain and simple. Not taking the extra time to learn theory will be wasted trying to figure out how to get a certain melody you hear in your head, probably not being able to get that sound. The circle of fifths has to be the greatest tool for musicians.
I'll probably get all kinds of flames/arguments, but I honestly believe musical theory is essential. I disagree that theory isn't needed to make really great music (although, there are a FEW exceptions). You cannot and will not change my mind.
TLDR, learn musical theory.