When is a programmable logical controller the best choice?
In my world PLCs are used to control manufacturing machines.

PLCs are used instead of microcontrollers for many reasons:  


They are modular.  In trouble shooting a spare processor, I/O card or power supply can be swapped out.
Technicians can get online with troubleshooting software and view/troubleshoot the running program.  This practice isn't really troubleshooting the program as much as checking to see what real world inputs are inhibiting operation.
PLCs can communicate with other vendors hardware through common protocols such as DeviceNet, Profibus or Firewire.  This allows almost plug and play integration with drives, temperature controllers and other hardware.
In large factory floors with multiple production machines, made by multiple vendors around the world, it becomes advantageous to standardize on common PLC hardware.
Integration with an HMI product becomes an important factor.  You must have some touch screen application to interface your controller to a person operating it.  Again standardizing on an HMI product becomes a benefit.  Wonderware, Intellution or WinCC are some common products.


The simple answer to your question is using a PLC standardizes the platform for Operators, Technicians and Engineers.