Dalaran is, at its best use, a support tool. Its a place for troops to martial, where Mages can focus portals and teleport troops into hotzones. Its something to counterspell any magical defenses Ogrimmar undoubtably has.
They could have just explained that it doesn't need to be 'flying overhead' to be supporting the offensive, and that such a thing would actually put it in danger of being destroyed.
However if there isn't at least one Alliance Gunship viable bombarding the city, I'm going to be a bit miffed.