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.
Rear speakers, generally of lesser quality?
Necessary for Rear speakers to be just as good as the main speakers? I had some extra main speakers put them in the back of the room. I think they sound a lot better than smaller rear speakers i used in the past. There is a lot of information coming out of the rear speakers via 5.1 set up. Why go cheap?