Yes, I agree that standard PLC have many functions but do we need all the function in a simple application? I doubt.

You TOTALLY missed my point.

ANY fucntion performed by a PLC can, of course, be replaced by a uC and interface circuits.

The ONE thing that is ludicrous is to make something with a uC and call it a PLC UNLESS the ladder logic software for the PC is part of the package.

There is nothing wrong with PLC control and there is nothing wrong with uC control, but if no ladder logic software is provided it is NOT a PLC.

Based on the above:
If you program in ladder logic it is a PLC, if you do not it is uC control. So, if end user programming is required, use a PLC, if not - and the volume of identical units justify it, by all means go ahead and use uC control.

Erik