Why Don't You Have a Yahoo! ID?
People have several different IDs to access their accounts at gmail, digg, technorati, webshots, bloglines, goowy, etc. Isn't there value to having one ID that accomplishes all that (Y! Mail, Y! Photos or flickr, MyYahoo!, MyWeb, etc.)
There are those who don't or won't bother to change their marketing settings (which you use to opt in/out out of third-party offers and Yahoo! communications). I would guess those folks see "unsolicited" email that they weren't expecting nor want.
What are the reasons that keep you from getting a Yahoo! ID?