The network stack!
VNC is useful is come contexts. I don't know that it will be useful for a PLC but it would remove the requirement for the programmer to install software on their own workstation. Level playing field, everybody uses the tools provided on the Pi. There could be some very informative screens available over VNC.
The concern I have is that Linux is not a real-time system. There are a number of notable attempts to make it so but I haven't followed them. This would be more of a problem for motion controls. Simple ladder logic is usually run as a super-loop.
I'm not terribly excited about Bluetooth LE but that's another area where the Pi might be preferred. It would allow cell/tablet control of various systems. Probably not those that could harm people but perhaps access control, environmental control and things like that.