Given the way sound - like other radiant energy like radio waves and light - falls off at the square of the ratio of the increased distance (2x as far = 1/4 as strong, 3x as far = 1/9 as strong, etc), setting up a 7.1 system is much more problematic than a 5.1 system, since the "sweet spot" for listening is much smaller.
With a 5.1 system, there's more tolerance in seating front-to-back, allowing public movie theaters to set up pairs of left and right surrounds marching down the side walls. Once you add rear speakers for 7.1, the distance front to back becomes more critical, leaving a very small area for seating.
I initially set up my surround speakers in the back of my theater, following the illustrations in my equipment manuals, until it was pointed out to me here that this was not how theaters are set up and movies mixed. I also realized that when the surround speakers were behind me - and balanced with a sound pressure level meter - I could seldom tell whether they were even putting out any sound without getting up and going over to them, since the pinnae of my ears were filtering out the sound from those speakers. So I remain to be convinced that even 7.1 is worthwhile. Electronics makers will happily upgrade their equipment to 7.1, since that makes the price easier to accept (kind of like the way restaurants psychologically justify the prices they have to charge to cover their rents and payrolls by giving us portions too big to eat.)
Think of 3D. I've seen two features in 3D: Avatar
. Neither impressed me with their use of 3D. In fact, I don't even remember any 3D effects in Tron:Legacy
! The only 3D that impressed me was in the trailer before it, for a nature documentary, where I found that I could actually chose whether to focus on the animals in the foreground or the background.
But in a dramatic film, the director can't afford to give the audience that freedom of choice. That's why shallow depth-of-field and devices like pull-focus are used to guide the audience's attention. True 3D would undermine the director's ability to control what the audience is looking at.
But manufacturers will force 3D capability on buyers of equipment in order to justify high prices.
If a friend asks you how to set up a great home theater without breaking the bank, there's a primer to help get them up to speed, in the form of at a site I just set up: http://PRO-Home-Theaters.com
. (I link from that site to these forums.)
It's not argumentative like this post (but this place is for those already deep into the hobby), but simply lays out suggestions for how to get started.