Instead of this, it would be preferable to have an option to not force a change of permissions. I would rather use the notices system to nudge members to correct their e-mail address, but not stop them from interacting with the forums. I can use the promotions system to move members to an alternate usergroup if I do want to limit things. The current setup is quite inflexible, which is out of character for the rest of the software - I like being able to customise things using permissions/notices/promotions etc... Of course, stopping e-mails being sent to the invalid address should still be enforced - everything else can be optional using existing methods.
Could this be considered as a option for a 1.4.x point release, as it's not a huge change - but will have a big impact on how the powerful bounce system can be used.