SM-3 is a kinda special case. Its not a multipurpose missile to shoot down planes, helicopters and cruise missiles.
You have a very specific threat, from a very specific high level US peer power. And you specifically want a capability to hit their high value strategic asset. Its very expensive, and built in small numbers.
For Australia, it is even more curious thing, because we aren't really facing the same threats as Korea or Japan or Poland. The kind threats we may be using it on, are perhaps even more high value and ambitious, think orbital. Or it specifically saying we are going to have an asset deployed to a region where it would be used otherwise (which with just 3 ships is pretty unlikely when the US, Korea and Japan have much more suited capability).
So for Sm-3 to go from a latent, potential thing, to a real blind extreme value asset thing is interesting. I imagine it could also be about southern hemisphere coverage. Its not an Australia specific threat, its about global capability.
With very elliptical orbits you can make space based assets very difficult to intercept in certain hemispheres. I don't know how many Aegis BMD ships are frequently deployed to the southern hemisphere, but its not many. Few would have the capability to track and coordinate such an attempt.
It would make the effort of protecting space based assets much, much harder.
I have heard that the MU90 has hard kill capability, when I google this not a lot of info comes up, which is not suprising. Some of it on the first page is me.
The MU90 / IMPACT Advanced Lightweight Torpedo, is a joint development between WHITEHEAD ALENIA Sistemi Subacquei (Italy, a FINMECCANICA company),…
www.naval-technology.com
But Hard kill isn't exactly a black and white thing. A WW1 biplane has air to air intercept capabilities, but that may not be relevant against modern threats. There is a continuum. I imagine low speed underwater drone threats may be more easily intercepted. Mu90 says navigation still possible in 3m of water. They may be able to operate as mines as well. But there is a big difference between a mk48 and a mk54. Its like a 50 cal and a 2000lb Jdam. mk48 is like ten times the warhead power and the mk54 would be more about making a small hole in thick plate than a big hole in thin plate (more like a tank round in that regard). Mk48 could probably penetrate a surface ship hull on kinetics alone, I'm not sure a mk54 would.
It would make sense why the RAN continues to persist with MU90 capability on ships. And why mk54 etc are just fine for P8 and Mh60R. There are other advantages of each type, so it may not be that clear.