They don't. The BS21 isn't in the same league as the RC-10. The RC-10 is a much better speaker. Better build quality, better drivers,better power handling, better sound.
If you need an inexpensive $50 pair of surround speakers get the BS-21. I wouldn't use the RC-10s as surrounds unless I had other Energy RC speakers in my setup. They're good enough to be used as main speakers too.
my mains are rc-70's. center for now is a jl audio one but will be upgraded to rc-lcr or was considering the pioneer sp-c21 (which is how i found out about the bs21's - researching the pioneer center). was just wondering if the sound quality was close between the bs21 and rc-10 since ive seen such good reviews on the bs21's
"I am a shark, the ground is my ocean, and most people can't even swim." - Rickson Gracie
Why would you get a Pioneer C21 to go with your RC-70's? You'd be doing yourself a disservice as it is not a tonal match. Get the RC LCR.
If you're trying to cut corners and save money you can use BS-21s as surrounds but I would stick with the RC LCR as the center for sure. You want your front 3 speakers to be timbre matched. The RC-10s would still be a better choice for surrounds since they are timbre matched too.