The only person who would be able to write something with "no knowledge of theory at all" would be someone who has sat down at an instrument for the very first time and had never heard any other music in their life. That first piece they wrote would be using "no theory at all." After that, though, they would know some theory.
Even Greg Le Bullshit knows plenty of theory; he just doesn't call it by the name that lots of other people do. But I'm sure that he knows that if he plays a song with chords that go "G - C - G - D", and someone says, "let's play it in the key of A instead," the new chords would be "A - D - A - E".
He's familiar with the sound of moving from G to C even if he doesn't know that it's known as a "I chord moving to IV," and he knows that moving from G to Ab sounds very different. And I bet if I played him a G chord, and then played him a C chord (without him looking), he'd be able to repeat it right away because he's familiar with that relationship and knows what it sounds like.
People that "know theory" simply know names for these types of relationships. But the most important thing of all is developing your ear. IMHO, it's much more important to be able to hear or sing a minor 3rd, for example, than to be able to write G to Bb on paper --- or to hear/recognize a V/V chord in the key of G than to be able to write A C# E.
There are great writers on both sides of the camp:
"no theory": Beatles, Cobain, Dylan (I'm guessing... I don't really know for sure), etc.
"know theory": Sting, Paul Simon, Rivers Cuomo (Weezer), BRIAN WILSON, etc.
I remember specifically hearing McCartney one time talking about "From Me to You." The song is in C and, for the most part, sticks to diatonic chords like C, Am, G -- though it does have F7, which is technically not diatonic to C, but it's a very common blues sound.
Anyway, the brigde moves to a Gm chord (I got arms that long to hold you...). And he said, "We'd never used that sound before ... a Gm in the key of C." And that's exactly what I'm talking about. He may not know that chord is a minor v chord, or more likely the ii chord in a brief modulation to/tonicization of F (Gm-C7-F), but he knows/knew the sound of it, and he could repeat it in different keys (evidenced in "I Wanna Hold Your Hand" and others).
And that's my point. The moment you start becoming familiar with the musical language (which is almost immediately), you're employing theoretical concepts. Knowing the names for these is just a personal choice. The most imporant thing is to be able to hear them and have them stored in your library of known sounds.
You can study all the theory you want, but if you never develop your ear, the theory knowledge won't help you except on paper. I met plenty of people when I went to UNT to study music that knew plenty of theory, but had very untrained ears. And, IMO, having untrained ears is just as much of a crutch (if not more so) than "not knowing theory."
Anyway, that's my two cents.
