Problem is when you have some kind of process that is inherently controlled not by the logic in PLC, but by some external system (either because the required data will not fit into PLC's data memory or because they constantly change based on some external bussines processes)
Reasonable architecture for this kind of problem would be attaching some server to the PLC as peripheral, but it tends to be done other way around. As for reasons for that I speculate that it is simply inertia of the typical PLC programmer which is then compounded by reasoning along the lines of nobody does that, so it is not tested and we will hit unknown bugs in the PLC firmware itself.
