Sogood51...Right on.
I would only add that the "right amount" of signal depends on the music in question, and the intended perspective of the listener. People who actually play in a musical groop may very logically prefer to be in the midst of the sound, not at a distance. I am told that some DVD-A discs actually permit you to select the "perspective" that you prefer.
There is a category of music, "Antiphonal" which REQUIRES that sound come from all directions. That is the way the muscians are located in a live performance.
Finally, IMHO, a solo singer is the best reason to have a center speaker. The soloist is a single sound source, and should logically be reproduced by a single sound source. The ability of a good set of stereo speakers to image a mono signal in space between them is amazing. But it is an illusion, easily broken by moving away from the "sweet spot".
When listening to a solo instrument, eg: a piano, I sometimes do the playback mono through one speaker. This replaces the recorded hall ambience with the character of my own room, as if my piano were being played.