Rear speakers, generally of lesser quality?

why not do a 9.4 system then?  

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. 

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

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

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.

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.

In stereo listening, the weakest pair will be the top level of your system. I think the same idea holds in multichannel systems. Weakest link and all that. 

I agree it is best to match surround speakers to the main set, just as matching the center to the left and right is important.

Despite this, I have been reasonably satisfied using good quality "full range bookshelfs" for surrounds.  I have had Proac 1SCs and am currently using Red Rose R3s.  They give me decent surround information, although I'm not set for immersion in a helicopter landing.

Why?

Size constraints.

Some of us do not live in McMansions.

I had the best sound in my theater when I used speakers that were actually used in the theater. That being said if you can afford to its best to match the loudspeakers when doing surround still some rear surrounds are designed just for this use and budget and room size are all factors. Most of the small all-in-one home theater speaker offerings are very much built to a price. And many consider good sound to be a very low priority.