In Melbourne the radar service is not terminated it continues to ground level because Approach control "owns" the airspace. The Tower "owns" the runway so you call them to get an aerodrome control service. i.e. a landing clearance. In Launceston the Tower controller is also an approach controller (procedural) and the Tower "owns" the airspace, hence the radar service is terminated.
Multilat's advantage over ADS-B is that it works on the currently installed transponders. It doesn't require Airservices to buy everybody an ADS-B unit for their aircraft. It also does not suffer from inherent GPS inaccuracies, downside is it requires more units to perform the triangulation.