Well I know a site which uses points and a shop, and with points you can buy themes, themes are around 5000 points, and themes are commercial, not a simple theme, really nice themes, which costs around 200$ in cash, so what is the problem on that site ,it is spamming..Indeed Free Domain is a good idea, but when you start offering something for points, people will start spamming... So think before about that then maybe offfering something for points.