As Larry Niven and others have pointed out, such a device (big laser) would make a potent weapon. Terrorists might wish to find a way to direct its multi-gigawatt beam elsewhere than the normal point of aim...
And what, he asked pointedly, do they do with the laser when it is raining and/or cloudy?
I recall one study, BTW, that indicated it would take 50 space shuttle flights a year to build even a modestly-sized space power platform. Uh huh. At $10,000 per pound, FOB earth orbit...I wonder a bit at the economic soundness of this.
--Boris