To all those concerned about the power beam going off target and frying the neighbours - the easiest solution (even if it is just psychological) is to put the receiving station in the middle of a large lake or other body of water. That way if the beam does go off target all it does is slightly raise the temperature of some water (probably by an amount too small to measure). As I said - psychological. The same reason why the Sydney Harbour Bridge has four large pylons that are purely decorative.
As for using it as a weapon - sure, you could. And in doing so you'll be shutting off a large piece of the power supply to your own people. I think the greater concern is physical security of the actual transmitter. Bit hard to put barbed wire and guard dogs around a multi-square kilometre object 36000km above the surface of the Earth.