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 that the planet relies on space travel for a significant part of its economy, which is pretty much te assumption of this whole debate). That planet in turn will want its own space fleet to protect its spacespace. When the two forces meet, you have space battles. It seems simple to me.
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 well as commensurate defenses. It isn't unimaginable that the destruction created by future weapons in order to penetrate future defenses would be immense. High velocity and mass will be the only way to do this... mostly in order to prevent deflection/destruction. You have accelerated a projectile to significant fraction of the speed of light in order to get it through the target's point defenses (think missile defense shield). Anything that can penetrate the missile defense system will have extreme destructive force, without needing a warhead.
Generally the point of a blockade/siege is to starve out the opponent so that you can eventually take whatever they are squatting on. Your opponent has a big weapon that you don't want to shoot at (you aren't looking to destroy what you are fighting over). Your blockade would be a sitting duck unless you destroy the weapon, and you significantly reduce the value of the planet/asteroid if you destroy the weapon.
In addition he also has perfect visibility due to the empty nature of space... he can see and shoot at you whenever he wants. If you intercept his transports, he shoots at you. If you stop to refuel, he shoots at you. If you start taking out his less valuable assets, he shoots at you. He can shoot your other assets too. Your space armada has to refuel at a fixed point in space... likely visible to his weapon.
If there were a fight, it would likely be very short and involve both sides having their logistical assets completely annihilated... Which ultimately would defeat the purpose of attacking in the first place.
Two things:
First, I don't think that huge kinetic weapons would be the ideal choice for this kind of combat. If I'm attacking an asteroid with a huge ass gun on it, I'd first like to have some type of EMP type weapon developed to simply shut down the tracking/aiming/power supply of the asteroid's defenses. Just the threat of an attack like that from orbit would probably be enough, as it would likely shut down life support on the rock as well.
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.
