Terminal airspace size
Larger Class D and Class C airspace around Australian towers means that greater diversions are required by aircraft avoiding the airspace.
I think I have followed this thread fairly well, but I don't think this has been raised by anyone. I really don't think this is an issue
Class D airspace around regional towers is
just an appropriate size for the use. I remember when we had 16nm radius down to the ground, now 6nm.
Why avoid the airspace? We do our best to facilitate all flights, arriving, departing, transit. It may be that going around the CTA steps / Control zone might end up quicker, maybe not. Never hurts to ask, just don't wait til the last second to call, give us a bit of thinking time.
It also improves things if you can make the initial call with callsign and advise if inbound or transit, wait for an answer, then go ahead with full details. This achieves a couple of things. You get the controllers attention, they know what colour strip to put your details on, their mind begins to add you into the traffic picture and it reduces repeated transmissions.