-------------------- I have plenty of experience in biology. I bought a Tamagotchi in 1998... And... it's still alive.
Registered: Apr 2005
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To be sure, there have been extremely few situations where we could have seen Earth's defenses in action.
In two of the movies, a super-entity neutralized defenses without a battle. In TNG, the Borg hit the Sol system, but the camera did not follow them: we only heard that an epic battle raged around Jupiter, and we never saw what sort of fight took place around Saturn (which the Borg were shown visiting). Hence, we can't say that nothing happened around Earth or that only three missiles happened around Mars merely because we saw nothing else.
In ST:FC, Earth defenses probably wouldn't have fired until the starships were out of the way. And the starships were more or less winning the fight already.
In DS9, a Breen fleet was cut to pieces so that it could barely pockmark a single city. I'd say there were defenses there...
It's just one of those Hollywood things. The Enterprise-D did have a big shuttlebay on the saucer, even though the camera never visited the facility. There were more than eight Borg aboard the E-E in ST:FC, and more than eight Klingons aboard Kronos One. So Earth probably had a dense swarm of orbital defense platforms, too.