Research In Motion says its BlackBerry email device is so critical that a court-ordered shutdown of U.S. service could threaten public safety and business productivity. The Canadian company is trying to avoid a possible injunction, the result of a long-running infringement case won by NTP Inc., a tiny patent-holding firm. In a filing Tuesday in federal court in Richmond, RIM argued that there is “exceptional public interest” in keeping BlackBerries beeping. And the idea of exempting government and emergency users from an injunction–which NTP has suggested–would result in errors, RIM said. [Somehow, I just don’t see it being all that bad.–Ed.]