Well, that depends. In some situations it could be the best use of the leader. E.g. if it's still early in a long-running spaceship or UN game.
I even used an MGL for FP recently in a quick and dirty Domination game, however, that was in a PtW game, where the FP is much more powerful while Armies are more or less wasted. In a C3C game I might have chosen an Army instead. But as it was, the FP allowed me getting a second core up and running very early, and that sped up the conquering of the world much more than an Army would have.
Yeah but that's different
In PTW you can abuse Forbidden Palace to wreck corruption
In Conquests it's determined by map and number of cities
So your 2nd best city will be getting it - I usually give it to the best shields/production