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.
What are your front 3 speakers?