Rear speakers, generally of lesser quality?

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.

Why?

Size constraints.

Some of us do not live in McMansions.

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.