The reason power chords are used more extensively in rock, usually with distortion, is this:
A note put through a distortion unit will sound like a major chord.
Let's look at why this is. We need to cover harmonics, scale theory and the properties of distortion units to do this. Here we go...
Harmonics are the multiples of any fundamental note. For example, an open A string at 110Hz will have a second harmonic at 220Hz, a third at 330Hz, a fourth at 440Hz, a fifth at 550Hz and so on.
On the guitar, you can produce the nth harmonic by lightly touching the string at 1/n of its length and plucking it. Touch a string very lightly above the twelfth fret and pluck it - you get the second harmonic, which is exactly one octave higher (see below). Doing the same above the seventh fret gets you the third harmonic, which is almost exactly an octave-and-fifth. You can also touch the string at any of the n equally spaced points - so you could touch at 2/3 the length, over the nineteenth fret, and get the same note. Of course, touching at 2/4 of the length will get you the second harmonic not the fourth (as 2/4 = 1/2).
You can demonstrate what is happening with a rope. Get someone to hold one end. You hold the other and start to swing the rope like a skipping rope at the frequency that feels most natural. Now try doubling, tripling or quadrupling that frequency. See the stationary points (nodes)? On the guitar, those nodes are where your damping finger is.
The common scale in Western music is made up of octaves (doublings of frequency) each divided into twelve equally-spaced semitones (although read up on tempering). The ratio between each semitone is therefore the twelfth root of two (hereafter referred to as r) which is about 1.059.
Solid-state distortion units tend to add odd nth harmonics (third, fifth etc), decreasing fairly rapidly as n increases. This distortion tends to sound harsh. Valve distortion tends to boost the even harmonics, which sound warmer.
Relationship between harmonics and notes
OK - so what note does the nth harmonic relate to? We need to find out how many semitones correspond to a multiple of n. This is:
x = log(n) / log(r)
n x interval
2 12 exactly octave
3 19.02 oct + fifth, slightly sharp
4 24 exactly two octaves
5 27.86 two oct + maj third, fairly flat
6 31.02 two oct + fifth, slightly sharp
7 33.69 two oct + dom seven, quite flat
...and so on. Usually, anything above the fifth harmonic is quiet enough as to solely form part of the sound of the instrument, distinguishing it from a sine wave, rather than being recognizable as a distinct pitch.
Using this, we see that a distorted power chord of C will contain C and G as fundamentals, G and D as third harmonics and E and B as fifth harmonics. This gives a power chord the feel of an unbalanced Cmaj9 chord. Adding your own third would muddy this further.
This also shows why distorted minor chords are even grungier than major chords.
This writeup is now online at http://tranchant.plus.com/guitar/power-chords