Before you edited your post stating that if we copied the US class D airspace we would face frequency congestion because the tower would be responsible for providing services to traffic up to 8.6nm from the airport.
This seemed unacceptable to you. I agree. Dick Smith probably agrees with you too.
It is unacceptable for a tower controller to be distracted with providing services to traffic that as you mentioned, would be outside of view of the tower at distances of 8.6nm.
If we copy the US system the traffic within class D is viewable at 4.3nm from the tower.
Its good to see that you agree then with reducing the size of our current class D towers at places like Maroochydore (13nm) and Launceston (20nm).