Not a bad idea... just an idea no individual strongly agreed with.
Again, I like Google... it's just that currently, if one wishes to post here, perhaps referred here by Facepunch, they have to create a 2nd account
(Facepunch #1, Ulysses #2)
If they wish to 'really' be involved in Mantis bug reporting, they must register another account.
(Mantis #3)
With the google moderation area, they'd be required to register yet one more account.
Though yes, Google accounts are popular, not everyone has one, and if they did, they may not wish to share that particular name/userid with forum viewers.
That, and I'd often end up pointing people back here anyway when a 'been suggested before or already exists feature' was suggested.