To contribute Finically to the revenue of the RTA, or what ever your states governing body is called.
Identification; If you see some idiot on the road doing something illegal or endangering life, they have a number plate that can be referenced to police or whoever... Bike riders do not.
The registration process could include some of the basic road rules they should obey, making it safer for everyone.
With out being silly about it, like kids riding there BMX in the drive or anyone riding on bike paths, go crazy, please, thats what they are there for. I am talking about the cyclist who use the roads to ride on as recreation or professionally training for sport events etc taking up lanes and often causing drivers to be nervous or make dangerous decisions to get passed them or whatever.... pay rego.
Not really an argument for it, its more my opinion, which I understand may not be popular.