Rear speakers, generally of lesser quality?

I’m not sure about less quality, but having a matching treble character and general dispersion and compression characteristics are important.

WHY we often get smaller subs though is usually due to size and money constraints. Not everyone has the room for 5 full size floor standers, and small surrounds are really convenient with subs.  Imagine getting super expensive main speakers for stereo, and then more than doubling it for surrounds  and a center.

Some manufacturers used di-pole surrounds and those were often even more expensive than the main satellites.

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.