Remember, this is just my opinion on the subject.
If an ad has no picture, even if it has a description (5'6", brunette, 36C-26-38, etc.), there is a possibility that more than one provider is sharing the same ad. (Of course, that would mean they would be sharing the same phone number also, but that is easy with services such as Google voice.)
Lets say 2 girls are sharing the ad, Girl A and Girl B. Hobbyist C calls number in ad, talks to Girl A and schedules appointment. Hobbyist C and Girl A meat and have a good time, Hobbyist C posts review. Later, Hobbyist D reads review and ad, phones number to schedule an appointment, but on this day Girl B is using the number. So, session is with Girl B, not Girl A, which is who the review is for. And Hobbyist D has no way of knowing that the provider he is seeing (Girl B) is not the girl in the review! If there was a photo, he could recognize that she is not the girl in the photo and therefore realize that there is a scam going on.
Basically, with no photo, you are more likely to be reviewing an ad instead of a specific provider!
Ad sharing is much more likely to happen with agencies, or dare I say, pimps (yeck!). This makes it even more likely for a scam to be taking place.
Unlikely to happen you say? Actually, this was quite common (as I experienced it personally) in the pre-internet days (1990's) when providers advertised through small classified ads in the "alternative" newspapers.
Again, just my opinion on the subject. If a provider does not wish to have a photo in her ad, that is her prerogative. I just feel that it should exclude her from the possibility of having a review posted here.
P.S.: It is also my opinion that a selection of "no" in the real photo category should automatically make the performance score a 1 - Rip Off.