Using Gravatar enables AB to ensure that, for example, animated images (which can be REALLY annoying for people viewing posts) can't be used on this site.
The golden rule (to avoid spam among other things) is always to create a new email address when signing up to new websites (or when purchasing something from a new vendor online). I know that I've used well over 100 email addresses so far (and it's probably nearer to 300), although only 5 are currently 'active'. Using the same email address for two different sites is simply asking for problems.
Chris