To follow on from what CL says,
Normally, all the valuable recyclable materials are harvested (copper from the wiring, good steel, etc) and all that's left is aluminium and magnesium. Not too good for recycling, I'm told.
Then, once sunk, it becomes a tourist attraction for divers injecting money into the local economy. It also provides scope for more advanced diving activities such as rescue diving from enclosed spaces (something that a lot of overseas agencies practice but find few appropriate venues).
It also acts as a FAD (Fish Aggregating Device) in the form of an artificial reef attracting fish for the local fishing population. Great pastime if you're into it.
As for the RAAF, they've got a hard enough time trying to find the money to keep aeroplanes flying let alone using a Harpoon Anti-Shipping missile to make it into a pile of junk on the sea shore where it can't serve useful (human) purposes... although I do agree we need to give the RAAFies some more opportunities to practice their craft.