That's up there with the "We'll ensure you don't share your username and password for our subscription service by printing your credit card info clearly on your account page".
A tactic I witnessed a content company do once. It was highly effective, but that doesn't mean I approve.
Of course I wouldn't like it, which is why my user name here and my email are totally unique {shiftyeyes}
On the other hand if users email was showing maybe they'd change their username a bit faster! :)