Good grief, I don't understand at all why it concerns people when other members refuse showing their real pictures as their profile pictures.
To begin with, it's everyone prerogative to use whichever picture they want as their website's avatar (profile picture), unless the picture is breaking the site's terms of service, of course.
Secondly, everyone deserves their intimacy and privacy, as no one can take that away from them. And frankly, I actually recommend and encourage members to take care of themselves and their privacy, and to think twice before giving out too much superfluous or private information over the internet.
Beyond that people may have myriad reasons of keeping their real pictures private, and all of them are indeed very justified and understandable. I cannot see the reason for asking members who take care of themselves and keeping their privacy to justify what they're doing as it is very understandable and even recommended.
Now then, you can have a small list of different reasons for keeping your privacy and yourself safe:
1. It's for your own safety. You need to feel safe and protected while you are surfing over the internet.
2. You don't feel exposed due to giving out your pictures and other different personal information that must not be given according to the site's terms of services *(yes, this is included in the policies and the terms of service)*.
3. If someone's will is not to give out their real life pictures, that is their full prerogative!
4. Pictures can be plagiarized and used maliciously against the original posters of the pictures. Again, that is all about safety and privacy over the internet.
5. Maybe people prefer using abstract paintings as their avatars (profile pictures). There's nothing wrong with that, at all.
6. The pictures can be violated in many different ways (Zalkari said one thing about them being used magically wrong, which is also one valid point).
So shortly, everyone has the right to keep profile pictures private.