Well, I'm reading the book Lucifer's Hammer and it's essentially an 'end of the world' novel brought about by this comet that strikes the earth in three different places. Two in the sea, one on land. The comet wasn't very large, either, just a 'hill' in their words, about a mile across, of dirty ice and dust.
Now, imagine a battleship exploding in orbit. Half a kilometer of solid, burning metal striking the planet's gonna do some serious freakin' damage. Sure, planetary shields and all that, but how do you stop something with enough energy to blast a crater the size of a small town? Now image that ALL of the debris from the fallen capital ship land in many different places on the planet. There's no shield strong enough to stop it all.
What I'm saying is that planetary battles need to be re-thought. Losing a carrier or dreadnought or whatever in planetary orbit is going to hurt. A lot.