Most of the "Playing" sites I've seen have been more directed at acts rather than relationships... They seem more like "Adult friend finder" and less like OKCupid... The profiles don't seem to say much about the person, just what their into. So, while I may enjoy playing with someone whose kinks line up perfectly with mine, If that's all we have in common, that's not much to found a long term relationship on...
I guess the answer to your second question is "no, not the main basis, but a significant part."