How so? He's taken 9 wickets at 2.8 rpo - neither average nor "easy to pick off" in this series. He's certainly the best option we have available at the moment, and a whole lot better than any spin the Aussies have chucked at us.
I am a big fan of the Duminator, but even I'll admit that passing the ball to him was a desperation move that paid off when Clarke was given a full toss and had a brain fart. "Possibly just over-pitched a little"?

Spin is much more effective when the ball actually hits the ground somewhere in front of the batsman.
In the past I would have argued that it is pointless to play a spinner if we don't have someone who is good enough. Harris is more than good enough, even the Australian commentators have been impressed. Duminy's spin is a "nice to have" but let's give him a few more overs before getting rid of a spinner that has troubled the (formerly) best team in the world.