I started off thinking that you needed to have a good image between each set of speakers; surround or otherwise. And this works for a single listener ... you can have a great surround setup with very few speakers for one listener ... the listening area being a single point, the MLP. As the more listeners are added, as the listening area expands (the box drawn around all the listeners) you need more speakers for all of them to localize a sound as coming from the same point. Imaging still works for the group of listeners in, or close to, the sweet spot. But people who fall out of it may find that sounds do not move smoothly from one place to another.
If you’re sitting right next to the side surround that speaker dominates. An object gliding from front to back may sound like it jumps from the front to the side, hangs a bit and then jumps to the back. Adding more speakers would smooth out that transition.
Now as far as how full range do the surrounds need to be ... well the general consensus seems to be the fuller the better. If a truck is rumbling by ... how much of that rumble is directional. It’s a compromise... I went with M5s for bed surrounds and M3s for everything else but I’m sure M3s or M2s would have been fine for all of them.
