Why are parallel fifths forbidden? Why should the only known (for hundreds of years!) polyphonie suddenly be not ok?
This has probably been answered elsewhere, but the reason is simple. Parallel fifths are avoided between voices in a contrapuntal texture because they make it sound as if a voice has dropped out. The movement of voices in fifths (or octaves or fourths) in parallel sounds more like a single voices (and has been exploited as such in orchestrations and organ design.) Counterpoint emphasizes not only several melodies but several independent melodies. (Even a long number of sixths or thirds, maybe 3 or 4 or more, is often avoided for the same reason.)
From my memory: In classical harmony theory having parallel movement in consecutive voices is usually (but not always) avoided. Obviously parallel 3rds or 6ths occur and are pleasant, counterpoint sounds better (to my ears). 5ths are a resonant interval, that is many harmonics between the two coincide. When this happens the combination of the two notes can seem louder than any other interval played with the same force. They actually are since there is more supporting vibration within the instrument (or between the instruments). The 5th often is described as strong or forceful. And this is generally considered unpleasant or severe. However, if that's the sound you want why not do it? Nothing is really forbidden in western music. Western music and harmony theory describe a set of guidelines for what was generally considered "pleasant" at the time (and that was likely determined by the Church).
The 5th is the only interval other than the octave where this harmonic convergence happens in the lower harmonics creating this effect. In modern music parallel octaves are considered quite nice (appearing in jazz guitar, Wes style soloing, and also in classical guitar pieces). Power chords are only a (1, 5, 8) and contain a lot of supporting resonance harmonics. Not sure if that is what you are referring to.
Music theory is a mixture of:
- Notation and terminology
- Scientific knowledge
- Style-specific rules and guidelines
"Don't use parallel fifths" is an example of a rule that is specific to certain styles of music. If you want to sound like Bach, it's a good rule to bear in mind (though even Bach allowed a parallel 5th here and there, IIRC). On the other hand, if you wanted to sound like a grunge band, you might follow different rules - like "avoid the V-I cadence" or "use distortion on the guitar".