In Die Hard 3 ("With a Vengeance"), the tactics of the bad guys constitute a denial of service attack on the police. First, they occupy everyone looking for a bomb in a school, then get them off their radios to slow down their communication. The same thing can happen with DRM systems. If your DRM scheme involves requests to a server on the internet (as most of them do), it could be vulnerable to a denial of service attack where all of your customers start requesting licenses at once. Or else someone with malicious intent starts directing a lot of traffic at your license server. And this exact problem has happened before.
Some people might say "so what, big crybaby, can't you get by without your games for a few minutes?" but with games being a bigger business than movies these days, and most of our entertainment consisting of movies, TV, music and games, plus representing most of our piracy too, server-based DRM has become more common and will continue to become more widespread. And all it would take is a large, sustained denial of service event to make people wonder exactly what good it does them as consumers.
Mokalus of Borg
PS - Then, of course, it would only take a week or two for everyone to forget it again.
PPS - Because we have such short atten- OOH, LOOK! SHINY!