User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-5436239-20130127223836/@comment-5436239-20130127231656

I don't think you are getting my point. I never said nor implied that all weapons used by ships are city destroyers. I'm merely saying that with military of logic and reasoning isn't going to fire on these PDCs because of possible damage outside of destroying the PDC. No matter what, if you fire from space, physics is going to kick in and blow the hell out of that area(Unless you are using useless projectiles the size of baseballs). If it's a kinetic slug, it's going to create a large impact. If it's an explosive type of ammunition, you have even more to worry about.

Sometimes they might just want the planet, but that is probably a rarity. Sure, it could be mineral rich, but if they stole this planet and our mining, I grantee you don't want to collapse this mine by firing around it. They want something on the planet unless this is total warfare. Mind you, if this is total warfare, they aren't going to give a rats butt hole about the cannons. They will focus on the fleet and units defending the planet, which in result opens them up to fire from the cannons. No matter what, these cannons are very valuable, despite their weaknesses.

Your scenario only works for something like the events on Horizon(Let's just say the cannons were working), with no defense fleet. Like I said, they are a fail safe.