His metal arms are stupid because they didn't even add anything to his arsenal of abilities. All the shit he does with his metal arms, he did just fine without them in MKII.
Yeah, but it was ultra dumb when he did it in MKII. He punches the ground and causes an earthquake? Come on. That kind of bullshit calls for an explanation. Either magic or tech will suffice. Johnny Cage and Kenshi are firmly in the magic camp, so it's nice that Sonya and Jax can take the tech stuff. It keeps the characters distinct and memorable.