I have 7.1 also... although not as cool as your guys setups i fear. HK AVR-325 with a Fluance SX-HTB set (5.0) plus a Fluance SV-6 set (2.0) plus an SVS PB-10. I was originally just going to get a 5.1 set but when I ordered my SX-HTB set, they were backordered, so I got a SV-6 set so I could have 2.1 until my 5.0 arrived. I hope that made sense.
Anyway... I think it adds a little, but you could definately do without it. I agree there are a few scenes where it's really neat to hear things right behind you (like a couple scenes in the matrix trilogy) but I could do without it.
I agree that if you can find some cheap set of speakers, try it out and see if you think it adds to the experience. I don't think it can hurt.