Ixixly, I'm aware of the SET that Oz allows - I worked there before & after it was introduced. Those limits for SET still don't apply to twins. It's been a while since I was in Oz, but what are the overwater requirements now? And minimum Wx. conditions compared to a multi operation?
Carrying a non-required crewmember is still one fewer pax or equivalent in bags or fuel. On international flight eg to & from the Bahamas I have to shutdown anyway to go through the customs & immigrations carry-on.