when I go to an event's page on Facebook, a flag/warning label appears telling me that Bob has been invited to the event, or that Bob will be attending the event (even if Bob is no longer on my friends list). The plugin would automatically search the guestlist for certain names. For people who don't want to share their space with, well, anyone regardless of the reason - but I'm thinking particularly of abusers, evil exes, etc. When you're in small communities, this can be a huge problem, and repeatedly typing their name in the search box is draining. And ironically, Facebook points out which of your friends are going to events - but it'd be bonkers to keep people like that as friends just for this function.
From an email I got: