Is Music Theory Important for Guitarists?

I would say it is important. Even if they say they didnt know music theory they did. Knowing basic chord structure is basic music theory. The greats knew that if they were playing a major chord there had to be a major 3rd even if they didnt know what the name of that 3rd was they knew it was not going to be a 3rd that was down a half step which would be a minor chord making it a minor chord. Even a chord progression, lets take a basic blues progression (1,4,5). First thing you need to know is your blues scales, 1, minor 3rd,4, augmented 4th, 5th, minor 7th and 8 (the octave). Lets say the didnt know that or could not read music. The great blues players that may have not known music theory but they knew that scale by ear and alot of theory is ear training and having an ear for what is being played. While they got along with out "knowing" theory I feel it is important for you to know, at least basic theory. It would only help you. If you knew theory you could play along with anything. Even just for fun you could through friend together and jam so much easier. You could sit in and at least follow along but knowing basic theory. I heard BB King say once that it was very important to learn how to read music and know theory. While he never really did anything to fancy he is one of the greats. If he says it it has to be good enough for most people haha.