As I said the inverted Stargate ship is just a place holder for what ever design people want to come up with but an inverted pyramid looks weird enough to fit in with the wedges and angles associated with the Empire and is a structure that's utterly impossible in our world, even if it were to be fixed to the ground.
Having it float into the air and off into space just ramps up the oddness.
If anyone had any doubts about my earlier suggestion of having R2 hack into the Imperial Bunker computer and shoot down their own TIE Bombers it looks like something similar is very possible in the real world :
http://www.boingboing.net/2009/12/17/hacking-the-predator.html