I'll guarantee most cyclists would break at least one road rule (I'm talking about a rule that motorists must abide by) on a regular basis (mainly in terms of running red lights)...
...as far as I am concerned, that kyboshes any other argument they might try to make.
To those cyclists who do follow every road rule, you have my complete and utter respect - my beef is with those who do not follow all the rules.
As for the argument about cyclists already paying registration because they drive a car, that silly argument doesn't apply to those who have a caravan, a boat, or especially a little box trailer (and some might only use them a handful of times per year). Use the road with x vehicles and you should expect to pay rego for the luxury.
What's the big deal? What is there to hide? $50 per year and display a plate on a bicycle - too much to ask? I suspect the problem might relate more to the abuse of the privilege.