Rear speakers, generally of lesser quality?

If you mount side and rear speakers on the ceiling, and but speakers that are fairly flat, it becomes a non-issue.

Take a look at something like the Vandersteen VSM-1, for instance.

If you are using the speakers surround sound it doesnt matter . The rears are  just for ambience.

Anything decent will work . Only ambience sound come from the rears.

When I bought my current house, there was a cheap home theater installed with in-wall speakers for five channels. I decided to take the in-wall speakers that were installed in the front and mount them in the ceiling slightly behind the seating positions. The rear in-walls were mounted on the back wall behind us.

Then I used my Revel F50/C50 speakers for the front L/R/C.  I am using a Marantz av8802 processor and Sunfire Theater Grand 7-channel amp to drive them. 

I've had much fancier surround speaker systems in previous home theaters, but this combination seems to work quite well - provide good ambiance and surround effects, while not taking any extra space in the modest size room.

The Revels work well in a theater environment. They have nice tonal quality and just enough detail to make dialog highly intelligible, but not so much as to make poor soundtrack recordings unlistenable. 

I use this system for tv and movies only. I am fortunate to have a separate 2-channel music listening room. 

why not do a 9.4 system then?