Maybe we're doing it wrong, but we use IPS for the same reasons that we use signature-based antivirus---as a backstop. Ideally, they wouldn't do anything, but we live and work in a less-than-ideal world with imperfect information security policies (and configurations), mobile users and visitors and students, and IT staff who aren't all infosec pros like you. And in that less-than-ideal world, both intrusion prevention systems and antivirus software mitigate enough risk to make their procurement, deployment, management, monitoring, and maintenance worth my while.