From the various people I've met on this site I'm going to say its not at all rare. I think the majority are switches that either don't realize it, or consider the title a hindrance in finding someone. So many Doms and subs want to find a one way or nothing kind of dynamic, so switches seemed to be brushed off. So to answer, I don't consider it rare at all.
My viewpoint is similar to yours. The intermingling of D/s & BDSM also muddies the waters. In terms of D/s, I'm not willing to submit and I hate being told what to do. Always have. Early in life, I found better ways to do things, problem-solve, and acquire expertise so that when someone was in a position of authority over me, I found ways to earn their respect or at least acknowledge that I had special talents they didn't possess. So with me, it's either I'm the Dominant one in charge or we're on equal footing, and I can't really give a percentage split on that because it's unquantifiable.
When it comes to BDSM, I don't consider that I'm bottoming when having intercourse or engaging in erotic activities that don't get edgy, so BDSM is really a small percentage of overall sexual activity. Therefore, with me, out of that slice of the pie, what percentage I'd choose to bottom is so low that it would be misleading to even assign 10-15-20%. It might be as low as 2%, which I could live without. Negligible, in other words, and when factoring in D/s, it can get complicated. Not to mention, my perception or ideological stance might not match that of a potential partner who has a divergent concept of how D/s & BDSM "play" out from his perspective.
Being deeply loved by someone gives you strength, while loving someone deeply gives you courage. - Lao Tzu
There is no remedy for love but to love more. - Thoreau