I would argue that registration pages should not let someone know if an email is in use - if a duplicate email is used in registration then the site should just send a quick email notifying the user that this email is already in use (with sensible DOS protection and email opt-outs etc.)

Your use-cases miss out one type of hacker/spammer/bad guy - the one who is only looking for you, they know your email, they want to profile you specifically.

Over the top/exaggerated example (with many implicit caviats!)...

Mr Smith has Mrs Smiths email - he uses it to try to register with a Dating Website - but hold on, it's already registered!

Now there could be 3 reasons for this...

1. infidelity

2. It pre-dates the relationship

3. A third party trying to cause trouble by signing Mrs Smith up.

I don't believe a company has the right to decide what data they hold on you is visible to the outside world - without expressly asking you.