The idea with Atmos and DTS:X is that they can use that 1980's technology idea of "phantom speakers" by making a sound appear to come from a location where there isn't a speaker, so having additional speakers, whether heights or ceilings, gives the system more places to combine sounds into "phantom" sounds. It isn't that these systems just play back a "ceiling channel 1" sound like traditional 5.1 or 7.1, so even if there aren't many audio bits coming from overhead, having the speakers make something closer to a "dome" allows for more three-dimensional placement of the sound in the room.
It is just a completely different way to look at audio playback... Getting out of the "channel" based mindset that we have been ingrained with for so long, and look at it as a single enveloping soundstage in 3 dimensions that just happens to be made up by multiple speakers.
With that said, I've heard early installer reports that they got a lot more bang for their buck by adding 2 front WIDE speakers and 2 overhead speakers to an existing 7.1 setup vs adding 4 overheads to a 7.1 setup. It was more enveloping of the main sound audio coming across the "ear height" sound plane, but still had some height channels to draw audio "up" into the sound space.
