This article in Technology Review (German) describes the emerging problems of infrastructural assets like power plants when they are connected to the internet for economic reasons. When SCADA systems are controlable from remote we will experience serious attacks in future.
But not only attacks are dangerous. The more connected computers and software are the more they like to communicate and start autonomous actions like software updates. This updates can trigger all sorts of weird effects like shutting down services (or the whole plant) or resulting in a defective system.
Not always it’s a good idea to interconnect. The more links a network has, the more complex it is to handle and monitor. So please let’s honor the air gap in such cases, ok?