I think the photo you use to show the world can go a long way towards presenting who you are as an online persona.
It is really only a small thing, but considering just how many people are out there on the web, every little distinction could help.
All in all I think a good photo of yourself (I say yourself because I agree with Austinstar's opinion) can lend a little more light to the persona you are presenting. A small part of a bigger picture, if you will.
For example: I use my pic because I like the way I seem to be deep in brooding thought which ties into some of my philosophical pieces of writing. I dont feel that I am not the same writer without said picture; but it does lend a little to the picture I am trying to paint.