Nick you have it exactly right.
The more speakers, the more immersive surround sound will be.
Overhead speakers very much add to that surround effect because sound is not localized to an exact point but rather extrapolated by our ears from various sources/echoes to determine a source point. Sound is a change in air pressure and air exists all AROUND our heads (the animated gif is a good example of how sound pressure moves and yet may not move).
http://www.indiana.edu/~emusic/etext/acoustics/chapter1_sound.shtml
Although the concept of Atmos adds a bit more of a minimal sound effect with ceiling speakers, it still fills in a very important perspective to which our ears (or any animals' ears) are attuned (i.e. sound reflections).
The best example is trying to find that airplane in the sky out in the backyard. It sounds like it is coming from the west but because of the distance from our ears, the echoes off so many surfaces, it could be coming slightly more from the east.
This type of effect can only be attained by adding more speakers in a HT setting.
I suppose future Atmos derivatives could try adding speakers to the floor as well but the ceiling seems to be the most obvious place to start.
I'm onboard the idea though that the number of speakers being introduced as some new "standard" for HT is getting stupid.
Most people i know still don't have a decent 3.1 setup.
Even some people i know who have a 5.1 system have no idea what Atmos is (when asked).
