People who are *really* into glasses also like to know the prescription of the lenses -- the worse, the better. And they don't like "fakes" (they can tell when someone is wearing planos by looking at the picture to detect a jump between the cheek and the cheeck magnified by the lens -- if there's not a big jump/disconnect then they know the lens is not prescription or not a very high strength).

Even if there are plenty of people who just like the "look" of glasses, the hardcore fetishists are interested in people with genuinely poor eyesight, and with some of them it even crosses over into a gimplike fetish where they like the idea of the person having such poor sight that they are handicapped and their vision is so poor that without the specs they will run into walls, have to hold their arms out to move around, etc.

Like gmartin said, it's also about what happens when the person takes the specs off and is "vulnerable".