As a mom who's been using a camera since she was about 5, I'm putting in a vote here to stop blaming "MWACs". I see it coming from males on and elsewhere at least as often and I doubt that moms have cornered the market on underpricing themselves or over-rating their abilities.
As for why some of us put a photo of ourselves as our avatar and a link to our website in our signature line, I'm with those who said we want to share them with other photographers. I have no illusions that anyone except another APUGer is likely to ever look at my website, but there's always the chance of someone passing it along to a friend (I've sent links to others before). My images also give them a reference point of what I tend to shoot. And having a photo of me as my avatar shows someone who they're "talking" to on the web (and it's an example of a good friend's photography and a rare occasion I got talked into modeling).