As someone hinted at earlier, the 'maintain AXXX' is usually written into local instructions at each location. Melbourne is 5000ft, Canberra is 10,000ft. It has to do with lowest assignable levels by other sectors and, in some cases, the ability for the tower to to auto release instead of asking for departure instructions.
If you are that concerned with the practice, go into the centre/approach room and ask about it. There will usually be an explanation in the local instructions to justify it. ATC are quite happy to have aircrew come in and have a look, plug in and ask questions. This is how we we all get better - by asking questions and knowing (to an extent!) the job of the other person.
Cheers,
NFR.