Many people are very quick to point out that hinting that an email exists in the system as part of a login error is a bad idea. What most people don't talk about is that nearly all of those systems that give vague error messages like "Either your email or password is wrong", will flat out tell you "That email already exists" on a sign up page, rendering that "security enhancement" on the log in page useless.
StackExchange is doing the correct thing for their use case. The usability gain is worth it, because if someone really wanted to figure out what emails are valid and being used, they'll just check the signup forms.
