Perhaps meteors original purpose was beneficial to the planet. If could be used to intercept and counteract asteroids or whatever that might hit the planet. It may have been necessary to use it for such a purpose originally, but then later it became a liability, since humans and Jenovas and whatnot aren't as peace loving as the Cetra and so it had to be sealed away. It may take an Ancient to harness the lifestream and actually summon the meteor (don't quote me on that) but humans have shown that they can work around that loophole when they created Sephiroth.