Pretty much sums it up. In a 6.1 setup, it's not a bad idea to use a center back there. I've tried both a center and a bookshelf from the same manufacturer/line, and I preferred the center speaker back there, but it will vary depending on your room, seating position and speakers.
I use a centre (single speaker) in my setup. Some others use a single regular speaker or even two regular speakers. The trick is getting a speaker with similar sonic characteristics to your other surround speakers for smooth transitions across the back and sides.