I have seen situations where dynamically assigned IP addresses for legitimate or new members are shared with ones in the SFS database. Same with user names, if a common user name like "John Smith" or something generic like that. Usually when these are flagged, they go to moderation and can quickly be affirmed or denied.
That said, for the most part I have found the SFS does a pretty good job at halting spammers, with Akismet less frequently but with near-equal effectiveness.