User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-5436239-20130127223836/@comment-5436239-20130127225639

So you are saying I wouldn't notice massive projectiles smashing my defense cannons? Yeah in space you have 100% accuracy, but not when it enters the atmosphere of a planet.

Again, if they are attacking the planet, there must be something valuable. It is very likely that these weapons are surrounding the target(Like Virmire or the Collectors attack on Horizon), and the blast radius(Plus the planet's effect on the projectiles accuracy) would result in damage to the target. This is true for almost every encounter. Exception: Total Warfare(No council race would dare do this) and the Reapers(Who may want to gather up all the organics).

It's like in the Persian Gulf War, some enemy bases were torn apart, but if civilians were around, they sent in teams to eliminate the threat.

Look at the cannons as a fail safe. They are very effective as nuisance(On occasion they might even get a kill) to invading fleets. They are for strategic purposes, not for eliminating a threat(They are still very capable).