In my opinion, you are missing out on a huge part of the overall experience without a good surround setup. The examples of movies and football games given previously are all very valid, but the one that really stands out for me is gaming. I happen to be a big fan of the Modern Warfare games, and I can't tell you how many times I have gotten the drop on another person in multiplayer mode because I could hear them approaching from behind on my surround speakers. It just really completes the experience for me, and I'm always disappointed when I play w/o surround, or on a crappy HTIAB.

My setup isn't the absolute best by any means, but I am very pleased with it. I've yet to be in a movie theater that sounds better.
It consists of:
Denon AVR-2309CI receiver
Polk RM7400 series front, center and surrounds
Klipsch Synergy 12" sub (rumbles the walls of my house on 1/4 max volume!)

one of these days...

sweet

nice...

cool

don't know why i bother

ooo... shiny

want!

sweet

shwiiing!