I agree with your friend. I also find it annoying when people don't post their photos.
I personally didn't feel comfortable posting my own picture, for silly reasons I  guess (mostly shyness), but some photo that you feel represents you is important. If you want people to interact with you as a full person that is. Part of the difference of this site, it seems to me, is that everyone is speaking as a full person which changes the fabric of the interaction. We are visual beings and having some kind of image helps in speaking to you as a real person.
I prefer to talk, and probably talk differently to someone who I know how they look. I understand many people are shy, or have their own reasons, but some kind of representative image helps.
There are layers of distance. It would be nice if we could all be in the same room together, but if we can't having some kind of image in mind matters.

Jeanne,
When I said I didn't like the idea of people not posting a photo to identify themselves Instead I should have said I was interested to know why it was so widespread in social sites. You may be right in some instances that it's simply a reaction to the photo ID world. By that I suppose you mean in the work place. Following that notion it does seem more interesting, cool and savvy to represent yourself by an avatar or some kind of well chosen sign or symbol when you have the chance. So it's a generational thing too, right? By the same token often little bio infomation is given on the profile pages. There seems to be no resistance to stating age, at least by decades. I can understand and accept all of the above, but I can't help thinking it's about communicating through cyberspace. Not trustworthy to give up your identity to people unbeknownst. 

Maybe anonymity comes with the territory because posting is about writing words not speaking them or seeing them spoken. Does it impact on style and content? Would you write the same way if your photo was on your profile page?