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

If I were an admiral commanding a fleet and saw my planets weaponry being bombarded, I would attack. I would also be thankful that my behind is being guarded by those cannons, and possibly make easy targets out of the attacking fleet for the cannons and my own fleet. They are meant for large ships and large fleets, things that won't be moving out of the way. A single ship is their disadvantage. When a fleet attacks, these cannons would not be the main priority. A defending fleet would be, or substantial ground forces. With Virmire you merely had a hopeful Saren. On Tuchanka, you have bragging rights+military might. Fear factor and effectiveness. If a ship is attacking a military base, but there are defense cannons used in the area, it could take them out. But with range it becomes less accurate as well. If you are attempting to take out a base, but aren't using bombardment in the first place(Likely for good reason, either because the base has required Intel, or in the case of Virmire, friendly salarians). Yeah, they could take them out, but not with the risk of destroying the objective.