Earlier quoted context omitted.
Why are you always supposing that ships would be firing at valuable planets, then debunking that? Just extend the idea of a nation's airspace to a planet's "spacespace." Establishing space supremacy around a planet would obviously be a useful military tactic. You can stop extraplanetary supplies and commerce (think the blockade from Star Wars) and eventually take over a valuable planet by starving them out (assuming…
Well, let's say I want to gather minerals from an asteroid. My primary defense would probably be to paste a big honking weapon on the asteroid and shoot anyone who wants to take over my valuable mining facility. If I put my defenses on something next to my planet, I'm an idiot. If he destroys my defenses, I want him to also have a high chance of destroying whatever he's attacking me for. So I build a big weapon as we…
Secondly, assuming that such a weapon isn't developed, depending on what you're mining, it seems likely enough that it won't matter what shape the asteroid is in. For mining solid materials, it would probably be best just to blow these asteroids into manageable pieces and send the whole lot through your enormous space processing plants for extraction and refinement. In this case, you'd need space fleets to protect your "planet crackers" and to protect your processing vessels from other companies, pirates, etc.