I don't think it's really a waste of space. It's a spawn point for the Annihilator after all.
Am I the only one who thinks that the lack of planes leads to the airport feeling like a waste of space? the airport takes up a significant amount of space in the map and yet there is no incentive to go to it.
The Annihilator also spawned on the helipad of the police station in northern Algonquin, and stealing that only gets you a one star wanted level which can be easily escaped in the helicopter, taking the one at the airport will give you a four star wanted level which is impossible to escape in the helicopter.
It's like calling the stadium in San Andreas a waste of space because you can't play major league baseball, some things are just there for scenery and immersion. I'd rather see a lot of buildings and structures with personality or at least a fake function than a bunch of stagnant buildings.
There is a difference there though, the stadium takes up a small area in a map surrounded by other things, the airport in GTA IV takes up a huge chunk of map and there is nothing around it but water.