I think for clarification purposes, we might want to differentiate between additional channels as opposed to additional speakers. If you are talking additional speakers, then I am with you here all the way.
Even though there is a duplication of sound in some of the speakers, whether or not sound design experts at some point start mixing sound in to additional channels in movies, if you have a bigger than average room, there is no doubt extra speakers can make a significant difference.
I agree with you. I have no doubt there would be a difference. Are you currently using any process such as Audessey with your current setup?
I have tried a couple of these Room EQ systems including the Audyssey process on my latest Marantz AVR(that I am using as a Pre-Pro)hooked up to two 7200s and an SMS-1. That is why I am able to drive all these speakers. The bottom line is I have never been particularly thrilled with the results(the process tends to be hit and miss) and it cuts bass response too much. It is well known that these EQ's can't read bass accurately below 50HZ and that even applies to readings from an SPL meter.
Some people love the process, others, like myself, got results that have been less than positive.
It is interesting to note that although they don't always come out and flatly state it, many of the speaker companies who tout flat and transparent frequency response(which is most, if not all), aren't particularly thrilled with people using Room EQ systems since they feel it negatively affects the original design purpose of their products and ultimately the sound.