I currently have 7.2.4 in my lounge TV/projector hybrid setup.
For a while I was reduced to 7.2.2 because I ran out of amplification on my AVR (Yamaha rx-A3070)
I now have an integrated amp with HT bypass ( Yamaha a-s1100) and so the system is back to 7.2.4
For many months (maybe 5+) however, I ran with front heights only with the rear heights disabled.
I am using Dali altecos for height duties.
When I connected the rear heights again at last things were much more complete, the Atmos panning effect actually works and immersion goes up a fair notch.
With just front heights you basically get just that, a higher 'wall of sound' that isn't particularly differentiated from well mapped surround sources.
It's nice however and would rather have it than not.
For immersion however you have to get four height channels and this is the consensus on many forums and what I found myself.
The helicopter Pans in scicario 2 in my room are are wow moment that I know only works with my four heights and not two.