Wow, lots of misinformation going around in this thread, I guess I'll step in before squeek does. He likes these threads too. The biggest problem with rear fill is that music is recorded in stereo. By definition, only left and right channels. When you put other speakers behind you, in the middle, etc, your no longer hearing the sound as it was intended to be heard. Music is set up so that you get a "soundstage". Essentially an invisible group of musicians in front of you playing the music. With speakers in their proper positions, you'll hear the music like your supposed to. Drums panning across the soundstage, definite space between instruments, not only left and right, but depth as well. (none of this is as easy to get as it sounds.) When you put speakers behind you, you get sound behind you, not where it was supposed to be. To have 1 get's rid of the other. So the vocalist that should be in the center of your sounstage is now coming from all around you. Sure that's envoloping, and it's great, until you actually hear how it's supposed to sound. Try listening to well recorded music on headphones sometime. Now, while headphones do encompass you in sound, in terms that it seems to come from everywhere, it's nothing like rear fill. Close your eyes and listen to headphones, the sounstage is basically inside your head. The center is in the center of your forehead, with the instruments panning at least to edge of your ears, sometimes further out! This is essentailly what your getting in car, just a foot or 2 in front of your face, instead of inside it. Imaging is part of what allows you to listen to your music, instead of just hearing it.
Secondly, adding rear fill presents phasing+cancellation issues. You have multiple speakers playing the same tones, at radically different distances from your head. Some of the tones will add together, some destory each other, creating gaps in your frequency responce, again, it sounds great until you realize what it's supposed to sound like.
Third, not sure who brough it up, but bass should not sound like it's behind you. The idea isnt' necessarily to mimic a concert, which does have speakers all around you. If anything, your trying to get a truly live performance, no speakers anywhere, just real instuments playing in front of you, that's what a stereo is shooting for in terms of staging. The reason a sub is behind you is because the frequencies that subs play are so low you can always tell where they are coming from, the wavelengths are too long. If you cross a sub low and don't have any major rattles, you can usually cross a sub around 60-80hz at a high slope and have the sub not localize at all. It will simply seem like the bass is coming out of nowhere, the same as your front speakers should sound.
In summation, SQ goes a LONG way, I've alluded to that a bit in my post, stage height, depth, etc. While few people will fully appreciate that level of quality, or spend enough time to get their car there, you can get the basics of a good stereo sound by just removing rear speakers and not running your sub too high.