it's all about attitude.
there is a similar debate amongst my biker buddies. in that community there is a definite difference between a Biker and someone who happens to ride a motorbike. you don't need to be out every day or riding to work in the driving rain to be a Biker. it's more about how you view bikes and how much of your time is spent thinking about bikes and bike related stuff.
in the same way i would say that there are Runners and people who happen to run. i have a mate who runs 5km on a treadmill in his garage every evening but virtually never bothers running outside. for him it's purely an easy way to keep fit that doesn't take up much time and when he's not doing it he never really thinks about it. he doesn't consider himself a Runner.
although judging by the amount of time he seems to spend looking at bikes on ebay, he is definitely a Biker. lol....