Absolutely correct statement, but I would clarify that to be missiles at extreme speed - Normal missiles - cannot be used normally. If you are looking at a typical explosive warhead, the heat and pressure will most likely tear the missile apart and may detonate it prematurely.Missiles at speed can't be used against a planet either. They will burn up.
But you can check my math in terms of evaluating the energy. A 10 kg projectile travelling at 500 km/s will deliver the kinetic equivalent of 600 kilotons of TNT. The scale of this is horrific. The bomb dropped on Hiroshima in World War II had a yield of 16 kilotons. You have a weapon potentially launchable from any starship turret missile launcher that can deliver such an attack. Even if it is destroyed in the atmosphere, that energy is going somewhere. You can end up with something like the Tunguska event http://en.wikipedia.org/wiki/Tunguska_event. Smaller if you have just one missile. But if half of that spread of 1,440 missiles I mentioned weren't diverted from the planet we could be dealing with a combined force of 420 megatons. Keep in mind when reading about the destruction at Tunguska that the object that caused the destruction did not survive collision with the atmosphere (it exploded before it hit the surface).
Think of a cruiser squadron delivering the explosive effect of the entire nuclear arsenal of the USA. And, because this is purely kinetic energy no nuclear dampers will help defend against it.
I mentioned normal missiles. Missiles intended for such an attack may be specially built around a single shaft of hard metal. Such weapons were described in the Night's Dawn trilogy as being pretty much a spear of tungsten. (For Traveller: crystaliron perhaps; with Imperial navy loads using bonded superdense).
The concept is well known and the military application of this in a space-based weapon (Project Thor) is under development as a conventional weapon useful for penetrating and destroying deeply buried bunkers. http://en.wikipedia.org/wiki/Kinetic_bombardment.
An orbital based kinetic telephone pole made of tungsten apparently isn't considered a weapon of mass distruction. A missile heading at a planet at 500 km/s is nothing but a WMD. Seems like something there would be treaties against - though how good the treaty is depends on how much a government needs the other party to follow it. (Ink on a page). Scary stuff.
(From reference material I have, a 5 kg kinetic missile 60 cm long would be about 3 cm in diameter. This would be 300 kilotons at 500 km/s but (being about 2 feet long) should fit in a ship based missile. Thinking a missile a meter long would need some sort of special launcher. We could do some math on a kinetic kill torpedo (maybe it splits off into multiple spears of 3 m length or more. Mass is a straight multiplier. A 40 kg spear would have the kinetic energy equivalent of 2.4 megatons and it isn't much of a stretch to imagine a torpedo carrying 20 of them. )