Rear speakers, generally of lesser quality?

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.

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’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.