“Why would I want to use an open-architecture PC-based system instead of a PLC for control?”

1. You can select the best of class hardware architecture for each system component (e.g.: controller, I/O, network, …). Proprietary PLC's often lock you into a single vendor's components and/or limit your flexibility.
2. You can take advantage of the increased technology at lower cost benefits in the larger PC industry vs. the industrial control vendor market.
3. You can extend the instruction set to meet your application needs.
4. You can more easily interface to other vendor's control devices (e.g.: motion cards).
5. You can easily and inexpensively tie into powerful PC networks as well as industrial networks.