That's literally my reason. Approval voting doesn't let you express any preference. If we had approval here in Australia, my vote for Labor would be exactly equal to my vote for the Greens. And I don't want that to be the case. I want my vote for the Greens to be worth more than Labor.

I think approval is likely to hard quite an extreme degree of moderating effect. Some moderation is good, for sure. Compulsory voting has a beneficial moderating effect. But with approval voting, it's likely that the Greens would never win a single seat, even when they're popular enough to win the election under IRV. Because all Greens voters will also approve of Labor, if they vote strategically in an effort to avoid the LNP winning. But some Labor voters might not approve of the Greens even if they would preference the Greens ahead of the LNP.

The other systems I mentioned disliking, "range voting" (one example of which is a system called "STAR") are worse, because they devolve into approval. If you could force everyone to sit down and vote 100% honestly it would be brilliant, but it's just as easy to vote strategically as FPTP is, by voting all maximum or minimum, with no nuance—i.e., approval. Only it's worse than approval, because some people might not recognise this and make their vote weaker by choosing not to vote strategically. Like how people in FPTP voting third party hurts their preferred major party.