For INXS I don't think it's even about someone fitting in or to take them in a new direction. I think it's about do they want someone to front the band or to lead the band.
They are middle-aged millionaire old timey established rock stars, with a large, bankable catalog of old hits, so I think they don't want someone to assume leadership, they're looking for a front man. That being said, since Marty seems to make it clear (or has been edited that way a la Burnett) that he wants to lead them in a new direction, and since his style is so different, they'd be crazy to go with him. The guy is really talented and he should go see if Nirvana is hiring or if Burnett wants to do a Rock Star 2:Nirvana series.
On the other hand if they are just looking for a pretty, talented front man, Mig is their man, that's his job. Pay him and he can be Freddie Mercury, pay him and he can be Michael Hutchence. The problem with that is INXS will then be officially relegated to being a nostolgia act. One band from the 70's who made a mega comeback has been Santana, and they did it by keeping true to their original sound and infusing fresh new talent (but on a rotating basis).
That brings us to JD. He has a good Michael Hutchence like vibe, is good looking but not model handsome, and has convinced some people that he's unpredictable and therefore a rock star-like edgy bad boy. His original song was tailored to sound like an INXS song. By going with him, INXS keeps the vibe yet infuses new talent. The editing has been mostly about J.D., his journey from wily Survivor-like schemer to reformed, great guy INXS loving tempermental artiste. I recall that throughout the season he was described as "rivetting". So Mig gets the votes, but JD acts like the public image of a true "Rock Star". INXS gets someone who "wants the job most of all" so therefore he can be manipulated and made to behave like a good "front boy", but still act edgy and arrogant to make rock fans swoon.
Everyone gets a record contract, and goes on to fame and fortune and lives happily ever after.