Actually, when you have the experience, the theory is automatically embedded in you. Theory was afterall born out of active music itself. Theories, music or not, are the trains of potential knowledge. I say potential because theories are not necessarily based on facts, but are the results of hypotheses, research, and opinions.
The first musicians did not start out with the theoretical component of music, but rather it was a medium of convenience. Bit by bit techniques, melodies, rhythms, structures and harmonies were put into notations. That also provided a standard platform for sharing knowledge, so one person could take another's music sheet and learn by himself. From there, groups like the orchestra came into existence.
Thus, to say a musician who has no knowledge of formal theory cannot be competent, is a fallacy. Remember, we are excluding the basics of theory, as by time a musician will already have them in his grasp whether or not he has had formal theoretical lessons. A self-taught musician will absolutely know the basics like a few scales and chords by the time he can actually play. By the time he can play spontaneously, he would have acquired the same knowledge as one who had studied music theory.
Of course, we are talking about musicians other than weekend jammers, orchestras, ensembles, big-time composers, music theorists, or music mathematicians since the topic concentrates on kick-ass Metal guitarists.