404 Titan,
Melbourne doens't need another airport because of capacity (it does for geographic reasons). What it needs is for the Dash8 and 340s to use EN, which would free up some slots and relieve them from the need to wait at the holding points at ML burning avtur. I have been told they want to use ML for marketing reasons, i.e., they want to be seen to be important enough to use major capital airports.
Walrus