[DarkMaster]

But the curse was that they would give aid in the great war until Sauron and all his forces were stricken from Middle Earth. And the plan to attack at the black gate was always a pretty inevitable battle, I mean they didn't have it planned until after the pelennor battle, but everyone always knew they would have to attack there someday before it would all end.

And the ghosts couldn't have attacked Aragorn, he's the king of Gondor and their only way to be rid of their curse (until the next generation of the king that is, but there's no guarantee there would be another, like Aragorn could die before having kid(s), or his kid(s) might not even bother with the ghosts).