You cant dodge every attack, and chances are in close quarters combat, he'd block with that arm being as its indestructible. In close-quarters combat, you wouldn't have a chance of defeating him, since he'll use that arm for every block, and since its impossible to dodge everything sans being the DC superhero Flash(or any other being with super-speed), he'd get you eventually with it unless you purposely exit close combat, which could be taken as metagaming considering our characters might not know about the powers of the arm, and thus wouldn't have any reason to play keep away.
And no, actually, in reality if I didn't know what the arm did, I would assume that he'd done that for use of his arm and hand bones as a form of claws and with better flexibility. By having a skeletal arm, you wouldn't have your own muscles restricting the movement of it, so it'd be faster and easier to use, and he could potentially have sharpened his own fingers into a claw. I would try to dodge it, yes, but more out of the same reason you'd dodge a guy coming at you with a axe, not because I though it'd have any supernatural ability(aside from being a skeletal arm, of course).
