I have never worked with a PLC , but I have read a number of articles on PLC. Microcontrollers!!! it is my life. I have heard a number of people arguing for importance of PLC over Microcontrollers. The following threads from some popular forums discussing the same.
What I have concluded from the comments is that
PLC is actually runs with a microprocessor or microcontroller based design which supports PLC instructions as its inputs and decodes it to execute it.
If you are good at programming microcontroller and your job to be done with the system is fixed, then it is unnecessary to use a PLC. PLC is a user easy option for electricians where they don’t need to understand all the circuitry and no need to train for every other processor we are using.
If you have employees who are unknown to microcontrollers and the programming language(or unknown to the specific microcontroller you are using) then it is tedious for them understand the things.
But the PLC is independent of the processor inside the instructions are standard.