nlagalle, you are correct in that 'many people will never take public transport to the airport, ever.'
However, if I recall correctly, when the Sydney Airport rail link was built, it started very slowly, with perhaps five per cent of passengers and staff overall using it, or even below.
It then built up to a 10 per cent share, and is now at either 16 or 20 per cent depending on who you believe.
If 15 to 20 per cent of Melbourne Airport's passengers and say five per cent of its staff used a rail line (bearing in mind that not all staff work at the airport: they may be in import/ export processing some considerable distance from the passenger terminals), this would make a huge difference.
So depending on how often trains ran and their operating hours, you might well be surprised at what a positive difference a rail line would make.
The way things are going, if it isn't built, the airport may be rather hard to get into and out of in five years' time for more of each day than is presently the case.