[Mvhsz]

I just rewatched the battle scene from the movie, and the wraiths took out a few trebuchets and a decent number of soldiers on the wall. They weren't effective against soldiers under any sort of cover, but picking up and dropping soldiers on the city would be a good psychological warfare technique. Gandalf is seen riding around the city telling soldiers to return to their posts.

And the communication and intelligence that flying wraiths would allow is no small asset. Part of the advantage of a walled city is a view of the enemy approach while hiding your own formation. Good areal surveillance eliminates that advantage for the defending side.

[HenryKissinger]

C2 Witch King: << Gothmog, I'm detecting a large movement 5 miles south of your position. They're not friendlies. >>

[wbl]

They are friendlies: the Witch King thinks the ghosts are reinforcements because they are flying Mordor flags. When they disembark the ruse is revealed too late.