Just a thought which no doubt has been mentioned many times before, but if a circle has a recumbent stone in it, is it more likely to 'suggest' that it was 'used' for something different to your bog standard circle without a recumbent, even if in the same area?
Nearly all RSC 's have central cairns and evidence of fires ,although other circles also have these features , the situation of the recumbent is also pretty crucial .