Ever since manufacturers started adding two more "effect" speakers to their surround receivers, taking it from 5.1 to 7.1 I've always commented to customers that it's just a way for them to sell more speakers. Do you really need 7 speakers and a subwoofer (that's the .1)? Heck, now you can find a/v receivers with capabilities for nine or even eleven speakers.
It's always been my opinion that unless the room warrants it then it's wasted money. I only advise on those added speakers in a very deep room. The money could be better spent on upgraded speakers. Besides, it becomes a real interior design issue.
CNET's Audiophiliac blog asks the question with their own take on the subject, Does home theater need more than 5.1 channels?