One slight correction to your "less idealistic view": It's not just (or at all) that one party believes they are the best to represent the whole, it's that they believe the other(s) are HARMFUL.

Ergo, disempowering that other voice is in everyones best interests.

While people can often do "evil" things (manipulative, etc), they usually do them for "good" reasons, at least according to their internal rationalizations. [citation needed :) ]