I've just posted on such a thread and in my view there are two problems. Firstly undoubtedly some punters become very defensive when one of their favourite girls receives a poor review and start casting doubt as to the credibility of the OP. Secondly, and equally intolerable, is when trolls and zealots respond by accusing those punters of white knighting or being fluffy, when in fact all they were doing, myself included, was commenting on how surprised they were that a girl who provides them with good service regularly, has apparently performed so poorly for another punter.
The exception to the two cases above is when a review is submitted that is so incredible and far fetched, that suspicion arises, particularly if the poster has previous form for benal and ridiculous contributions in the past. An example would be when a review comments on a girl's appearance, mannerisms, or the way she provides certain services, that in the view of the regular punter, are unrecognisable.