Ask HN: Mandatory use of 3rd Party IDs (Facebook Connect, etc)?

I noticed that 750words.com doesn't have an independent account login/creation system; you can only use it with an existing FB, Google or Yahoo account.

This is the first time that I've seen an app force you to use a 3rd party account. While it would certainly reduce account-related code, it could also have unintended effects. For example, on a social site, this lack of anonymity might result in nicer user comments.

What do you think about forced use of 3rd party IDs? What kind of sites would benefit from it? Is the trade-off worth it?