Why is this optocoupling circuit useless?
The entire point of the a optocoupler is to allow electrical separation of a circuit. It's literally a led, a little tube, and a phototransistor. You could make one yourself. You don't have to share a common ground, but you can if you want.

As for a short on the phototransistor size, for most circuits, especially low voltage low current ones you have nothing to worry about. Unless you connect a high voltage high current line that way exceeds the optocoupler's absolute maximum specs, enough that they cause the optocoupler to melt/explode/catch on fire.