I don't have any good reasons that it's done, I just have the ones people give me. I think it's an idiotic practice. If you're going to switch to a new system because you like it, maybe you should actually utilize the advantages that made you like it?
Reason 1 is usually some combination of diversity/keeping the players honest. Facilities are often going multi spec, with multiple PLC brands in play. I've seen situations where a new/retrofitted facility was split in half. Same process on both sides, but one integrator using AB and one integrator is using Siemens, but has mostly AB trained guys. In theory, forces the integrators to play fair, forces the component vendors to play fair; in practice it's a mess.
Reason 2 (following from reason 1) is "we want the same code no matter what PLC platform we use". Understandable from a maintenance point of view, I guess, but it means you're either essentially stuck with the lowest common denominator or you have to bend a system backwards to make it act like the one you really wish you were still using.
You become an engineer so you don't have to deal with politics and interpersonal junk, and then find out that it's worse now because everyone else was hoping the same thing....
