I honestly believe that music theory is descriptive as opposed to being prescriptive. Music theory has changed over the years to account for the way that musicians compose. In other words, the "rules" of music theory were different for Haydn than they were for, say, Wagner (although many remain the same, of course). I'm not saying that Wagner flounted music theory...he was aware of what he was doing. But, in the end, he composed his music regardless of what theory said he ought to do (I'm thinking of the "Tristan chord" in particular).
More to the point, especially in metal, the world of tonality has been kind of blown wide open with chromaticism and (over?)use of the tritone. While a knowledge of theory might help a metal musician, being able to tell what sounds good and what doesn't is probably more important. Also consider, and I don't mean this disparagingly, that Mike pretty much plays his music live like he does on the album (with some notable exceptions, of course like Closure and UTWM). Once he learns the part and knows what the solo will be, who cares about theory? He isn't really put into a position where he needs that kind of knowledge at his disposal, you know? Again, I'm just calling it like I see it and mean no disrespect to a guy whose music I love and who I think is a really great person.
