Will a speedlight double as a slave flash?

If I get a good Canon speedlight can I use it off camera with radio triggers? I had considered the yn-560 for off camera slave but then I saw a guy at a school dance, a pro, part of the staff that sells pictures after the dance. He was shooting a Nikon. It looked like a D300s, but on hotshoe he had his "real" Nikon flash and was bouncing the light off the ceiling.

My point is that if I can do everything with a "real" Canon speedlight then why limit myself to a slave only flash? Can't I have both with one device?

Yes, the canon 430 and 580 series speedlites can be used in slave mode. You may not need radio triggers if your body provides it, such as the 7D.