Arduino is definitely not cheaper. I've installed and programmed several LOGOs, and it's easy as hell. It's a compact package that already contains, in itself, enough inputs and output that can be used by i.e. electricians. An Arduino requires you to take the soldering iron into the hand.

It is just a matter of where it is acceptable or not. If one hour of downtime will cost 100 of LOGOs, production managers won't take a risk with an Arduino not rugged.