I don't see how it will save that much time for the average business traveler.
Firstly, the slower cruise speed will surely negate some of the time saved on taxi out and holding.
While Essendon is certainly closer to the Melbourne CBD the travel time on the freeway between YMML and YMEN is not that long so it's hardly a huge time saving.
Bankstown on the other hand is a loooong way from Sydney CBD along the dreaded M5 East which is bumper to bumper from 3pm till 7pm most days as best I can see. You would need to allow an extra half hour on the journey to YSBK than to YSSY from either Sydney or North Sydney CBD's.
But the biggest issue I see is the lack of frequency. You can't just turn up and go. You must have booked for a particular time and if you miss that service it could well be 3 hours until the next one. And knowing this, would you dare turn up to Bankstown 15 minutes before the flight? No, you'd have to plan on being there early as the unpredictable traffic situation could turn a 10 minute delay into a 3 hour delay into Melbourne.
On Virgin or Qantas it barely matters if your meeting goes 30 minutes over time and you miss your flight, there is another one in 30 minutes or at worst, an hour later.
With only a small number of flights a day and long gaps inbetween the convenience of "turn up and go" is not really there. That is a huge advantage the incumbents will have over this operation when it come to saving time.