I've always distinguished between "groups" - such as picture taker/photographer, piano player/pianist, biker/cyclist. I guess I'm thinking that to me there is the impression of more skill/experience/professionalism in the latter over the former. (That's not necessarily implying the former is inferior.) So, in my mind a "biker" (in the bicycling sense) is more of a recreational rider, or a family out on the MUP on a nice Sunday afternoon, while a "cyclist" is more of what we call an "avid" or "serious" rider, doing longer distances, faster speeds, and going out in less pleasant weather. Interesting topic. Never did I think I'd get into a linguistic discussion on a bicycle forum!