Now that I've been to a couple of the saunas that do post pics online (East and Supreme), I can say that it definitely helps gauge the talent level before going and in certain instances, there may even be something that catches your eye and entices you to go there. Case and point:
After visiting Rio looking for something exotic and different (Mongolian, Russian, Black, etc.), waiting in the lounge for a couple hours and then being told that they only had 1 Russian who had already been taken (after I specifically told the attended to let me know when they were available!), I left and headed straight to Supreme. I remembered that they had a number of Western girls on their website. And low and behold #588 was available! Her picture is very accurate except I would have to say she looks even better in person. Same thing with the girl I had from East (90-100% accuracy but better in person).
I'm not sure what these places have to lose/hide (other than the girls' privacy, of course) by not showing their goods or gain by using inaccurate pics.