Another thing to keep in mind with playing power chords and removing the third is science. If you grab an acoustic guitar and pluck an open string, you can hear the fundamental note (the root), plus a myriad of overtones. I am not talking about other strings ringing sympathetically, I am talking about one string by itself. It's difficult to hear, but it's there. As it turns out, those overtones ring in frequencies that are multiples of the fundamental note. The fundamental and the fifth are the most distinguishable notes you can hear (followed by major third and flat seventh).
Conversely, when you play a power chord (an interval of a perfect fifth), you are basically amplifying the root's natural tendency to call for that overtone. Like I said, one rings in a frequency that is a multiple of the other, so even though the frequencies are not the same, when they ring together, the waves move in such a way one pattern is perfectly contained inside the other, like this:
We call this consonance.
On the other hand, when you play a more dissonant interval, you get clashing waves that sound like they are trying to cancel each other. There are degrees of dissonance. There are intervals that are incredibly tense sounding (diminished fifth, flat ninth) to others that are not so much, even considered kind of consonant (thirds, sixths). These degrees of consonance and dissonance and the way we approach them are what makes music interesting!
And when you engage a distortion pedal, you are basically magnifying and modifying the interaction between those notes. So, when you play a third (either major or minor), it is going to sound different. It's easy to start sounding muddy and undefined, which is why we stick with power chords! They're open sounding and in your face. They are awesome!
I suspect Mr. Grissom's motives behind avoiding playing the third at a clinic are also related to the fact that the crowd is probably very heterogeneous and he wants to be as inclusive as possible.
