I've resumed my music classes and once again, they've told us to avoid using parallel fifths in our compositions. This philosophy has always baffled me because I really don't hear anything bad or wrong with parallel fifths. In fact, I have two compositions that I wrote specifically to demonstrate that parallel fifths don't sound bad. I think they can sound bad if you position them poorly, but then, any progression can sound bad if you position it poorly.
So, I've been wondering--for what reason are parallel fifths frowned upon? The best answer I've been able to come up with is that the relative strength of the fifth can make it sound like you're changing the tonality (i.e. the key) of the piece. Can any of you more experienced musicians offer some insight?