I run risers w/ 10 degrees of sweep, it's hard to find a flat bar with decent sweep. Even the Surly 1x1 bar is only like 7.
Also, if you have wheel clearance, it's easier to get bar clearance for barspins with risers. You need a more dramatic stem angle if you use flats.. Aesthetics, yeah, but, yo u know.

Zombie Carl- There used to be Kalloy risers on ebay in red, purple, and blue. I haven't checked in a while, dunno if they still have them.