In our overly paternalistic police state Down Under, 2 plates are mandatory and strictly enforced because of the millions of dollars in revenue raised by fixed, mobile and temporary speed cameras which use the plates to identify the poor sucker whose turn it is to donate to the State Government coffers.

The speed cameras have been shown to be innacurate time and time again and there are several group actions being taken in the courts to have thousands of fines recinded, but for the average Joe it's not worth the hassle of fighting it in court so we just pay up and hold a grudge until the next State election.
Motorcyclists have historically been the only ones exempt from having a front plate, and the Government has a big fight on their hands with the Bike Lobby trying to make them comply.