rollin:
It's been my observation that a number of people who are into spanking specifically are switches. The same cannot be said for BDSM in general. Maybe it's because spanking can be about sensuousness and physical sensation and BDSM is about power exchange.
I guess it depends where you are. In my geographical area in England, the vast majority of people in the general BDSM scene are switches, but the ones who identify as only 'spankos' mostly do not. Or if they do, they only play the other side in private, and not at parties etc.