The interesting thing is that what happens without the safe harbor is not a known quantity. The safe harbor is like immunity, but lack of immunity isn't the same as liability. But in order to find out, somebody has to take it to court, and the entities small enough to have not registered a DMCA agent will tend to settle or be bankrupted by the litigation costs before the issue makes it to an appellate court.
Yeah, it's not something we're likely to see put to the test, as if you can afford the legal bills to fight a copyright action in federal court, you surely can afford the small filing fee to register a DMCA contact in the first place in order to fend off the likelihood of much larger fees later.