IMHO for movies surround makes sense, less so for music where, again IMHO, to be realistic the surrounds would be more for ambience. I could buy the idea that the center channel could benefit in solidifying the center image though. (though I have to say that I get a pretty damn good center image from my 2-channel main speakers).
I personally do not really want to hear instruments coming from behind me, I can see it from the viewpoint of album production being akin to crafting a piece of artwork, where sound effects coming from the surround channels are part of the "big artistic picture" for lack of better description. But for me when I listen to an album at home I prefer to have a realistic "concert" perspective, meaning all the main sound in the front channels and the rears just for conveying the reflected sound and ambience of the place. Maybe then the acoustic differences between a ampitheater, church, and small club could be more accurately conveyed. But here, I wonder about the effectiveness of this when the final surround mix will be played back in a huge variety of environments and rooms with widely varying room dimensions and speaker placement. Just thinking out loud...