Nick has an interesting point. Being able to aim your speaker when you have a low ceiling. I'm using a mix of height and top surround speakers in my setup*. The heights are M3 bookshelfs and the tops are M3 on-walls. I passed on the M3 ceiling speakers for a few reasons:

1) Wanted to be move them if needed. This included an option of being able to setup my screen on the long or short walls.
2) Didn't want to put a hole in my ceiling (like Nick ... there is a lot of expense which went into soundproofing).
3) Ability to aim the speakers (heights on a FMCB and I was not quite sure how I was mounting the tops but if Nick vouches for the FMB for the on-walls that may be the way to go).
4)Transparency was the only other reason but as my post states (I'm not sure it matters)



*The mix of heights and tops were carefully picked to work with ATMOS, DTS-X, and Auro-3D.