I guess by default not showing the email address to the public would be the best way to go.
That's what I'd like to see. Perhaps some people just aren't noticing the Email display status right away. Some people may be new to the internet, or may be from a different, more friendly for lack of a better word, sort of culture, and want to, or think
they need to, appear more personable. And some people just want attention, "Plz email me for I have wonderful ideal of chemical venture to make much money recharging of most expensive ink cartiges." And I'd like it if that person were obvious, so I could ignore them, even if they're not running a 404 scam.
A while back people noticed I'd blocked personal messages. I did that because some high school girl sent a PM response to me -- She'd asked a bunch of "gimmie gimmie" questions, I'd done my usual response that tries to lead to some useful forum dialog, then she just PM's me with her real name, personal email, that's she's 15, and lives in some state in the US. I was almost stupid enough to PM her back, telling her that her age, gender and location aren't important, and that science dialog should happen on the forum posts -- but I, fortunately, stopped myself. A parent might have seen it, and not being very net savvy, might not have figured out I didn't initiate personal contact first, or, indeed, even have cared who communicated first. So you see, the personal contact, isn't often as good an idea as people might think.
This place has a high school forum, and these kids are quite notoriously dumb with regard to personal info, posting links to their Myspace and such. And parents and municipalities are likewise notorious for not caring about the specifics when problems arise. I don't run a web forum, so I'm not thinking hard about the specifics in this matter. But I'd hate to see this board replaced by an apology letter from Mitch, and a plea for funds for his sudden, unspecified, legal support.