IP logging doesn't help much for automated abuse of abandoned accounts (and if you reread the comment you'll see I mention that :P) but it can serve as a deterrant for non-automated abuses. Someone who finds the abandoned account of someone they think is so-and-so, with whom they have an ongoing internet slapfight, might think twice before commenting with the latest round of abuse if they're confronted with the "your IP address is being logged" message. (Those messages are, IMO, like the little sticker advertising the home-security system homeowners stick in their window/put up on a stick in their garden/etc.)

(also: *sends you buckets of flame-stopping goop*)