A lot of netizens nowadays consider their email more private than they may in the past. Email is niche social networking, more a place for business.
I don’t consider my email address sacred, and do hand it out often, but I don’t like handing it out to comment boxes. Weird boundaries yeah.
Handing out your email lets sites notify you of replies. I mean, maybe you don’t care.
It can also be used as anti-spam. I mean, its not hard to invent a fake address. I use a captcha to prevent spam here anyway. I also invent fake email addresses myself as a human.
Its the default option in WordPress, to ask for users emails. But I turned it off, or made it optional. Leave a comment if i forgot!!! in the mean time Consider making emails optional for yourselves, webmasters.