A solenoid isn't just a switch, which is a passive device and (assuming the switch contacts are in good condition) draws no power. A solenoid though is an electrically operated device, and that means it draws power to operate it. Drawing power means it will generate a certain amount of heat. If it isn't designed to be activated permanently, it may well overheat and burn out.
Having said that, surely Subaru must have designed it to operate for hundreds of miles in FWD mode. I've not seen any restriction on mileage while in FWD mode. (I'm not advocating leaving the car in FWD mode unnecessarily though.)
BTW, VDC equipped automatics don't have the FWD mode. It appears the VDC is supposed to take car of it itself, they do have an almost full-sized spare wheel which must help too.
