I don't buy into this argument. If someone doesn't have the knowledge and motivation necessary to use another platform, they aren't going to be making meaningful contributions to your project.
It might mean less bug reports from users who find bugs, but aren't deeply affected by them.
The upside is a lot less garbage in your issue tracker.
You're forgetting the users that are tired of signing up for one-shot sites to do patches or drive-by bug reports. The actual value of the sign-up process is so little that the incentive to contribute lowers. It's another set of username/password to manage. Even if you're unwise and depend on a password app, it's another thing on the list to manage.
OpenID was supposed to fix a lot of problems and doesn't appear to have, at all.
Anyway, there are lots of people who are not only knowledgeable enough but also skilled enough to triage bugs, but don't want to go through the rigamarole of a new account, e-mail, clicking a verification link, reading the bug report guidelines, etc.
Putting walls up to contribution is indeed a good way to get fewer people contributing. There's no guarantee the ones stubborn enough to still report bugs are reporting any better, though.
It might mean less bug reports from users who find bugs, but aren't deeply affected by them.
The upside is a lot less garbage in your issue tracker.