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I've even heard of people writing a bot to do this across hundreds of sites; it's very interesting to see which C-level executives have been using their work accounts to access sites they shouldn't have.
The obvious advantage of having a GMail or Outlook address is that only Google or Outlook have to know who you really are; the address could be anything you want and doesn't have to be related to your real name. In addition, you can do as I do and have multiple addresses -- not the "throwaways" which people often speak of, but simply different addresses for different online activities -- so that if an account ever got compromised, there would be no way to link it to any other account.
Email security and privacy is a hard problem to solve.