C'mom guys. Australia is not the USA.
In Australia, you are the tail and the government and its agencies are the dog. Private citizens exist to be told by government agencies how to run every aspect of their lives, so as to make life easier for the government agencies. If it's going to make it easier for a government agency if you file a flight plan, it should be mandatory.
Next thing you'll be claiming that your taxes pay for all those RAAFies and their toys at Williamtown.