That assumes they want to come back and update. I may not want to fully block, but instead want to just restrict from certain areas. Doing so leaves them the ability to PM an admin if they're having e-mail troubles (yahoo and aol frequently block). If they marked an email as spam, that should move them into a different group entirely.
It also gives me the ability to do cleanup. If they bounce and get moved into that group, I can do a filter to see if they have any content items. If their content count is zero and the account is over a year old, then I would just delete the account entirely. I send birthday and holiday emails not because I'm particularly jolly, but because I want to see if the email addresses are valid. Having bounced users moved into a group helps me keep the user database tidy.