Well, perhaps it's a case of different strokes for different folks. A factor, also, is how far apart each speaker is, and how far you are from each speaker. If they're all bunched together every two feet, and you're only a couple-few feet away, then yes, it's probably overkill.
I'm about 7+ feet from each surround speaker, and each speaker is roughly 6 feet apart from each other, so to me that creats enough space for a wide soundscape. Otherwise my two surround speakers (albeight QS8s are great) are 15 feet apart from each other. And they do a good job, I'd go so far as to say most of the job. But there are times where there's a sound RIGHT behind you that speakers 7 feet to the left or right can't duplicate.
My 2 cents is, I can justify the spending of two more speakers. First, you'd never get to play 6.1 discrete movies. Some may say that's overkill. That's fine. 5.1 is really great, but I like 7.1 better. It really comes down to a matter of opinion.
The bottom line is, NeverHappy, is that I might have agreed with you, up until a week ago. Most movies ARE in 5.1. Though some are in 6.1 and 5.1 EX/ES movies are made more for 6.1 and 7.1 systems. This sort of kept me on. But again. 5.1 was generally enough. Until I played Master and Commander in 7.1, chapter 4. I heard sounds fill the room like I hadn't heard before.
Most people who already have a newer 5.1 speaker setup probably have a receiver that'll at least do 6.1 or 7.1. So, to all you out there on 5.1 and are curious...go find a friend, or look in your garage or basement for an extra speaker or two that you have or can borrow and test it out for yourself. It's free. Granted, if they aren't a high quality speaker it won't sound the same as if you have an Axiom back there. But it's just for fun anyway.
But I will go on the record as say, "Yes, 7.1 is a negligable difference over 5.1."
But I did it because I could, because I wanted to, because I thought it'd be fun, and I thought it made a difference.
I'm sure the 5.1 vs 7.1 debate will go down in home theater history up there with good vs bad speaker wire, powerconditioners changing your sound and picture, and speaker break in...
To each his own. Enjoy the sounds!