Ten years ago, having a single, unique, permanent and publicly-available email address was seen as a good idea; it gave one an online identity, a convenient means of contactability. That was a more innocent time; a time when the internet had, until recently, been a quiet, friendly academic/research community network, home to nothing more hostile than Emacs-vs.-vi flamewars, and some people still chose not to put passwords on their accounts. Then came the carpetbaggers and chickenboners and script kiddies with their spam-sending scripts and email harvesters, and it all changed.
The reasoning behind Gravatar appears to be stuck in the pre-spam golden age of the internet, where an email address is something you publicise rather than hide, and the idea of letting untrusted strangers (or their web sites) have your email address doesn't set off alarm bells. Which is why it's probably doomed to failure.
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