[MattSayar]

How about for websites that give you some functionality without a verified email address? At that point, you can't let a user dink around if the address is in use.

Granted, this doesn't apply to eg banks, but there's plenty of websites where this could apply.

[JimDabell]

Don't ask for their email address then. What's the point in having an email address that you have no idea if it's correct or not? You might as well ask them to put in a random string of characters.