You'd end up with some nonsense of forwards running through and trying to leave an arm trailing behind so there was some overlap with the defender, then defenders would end up taking some odd body positions trying to prevent any trailing body part being able to be taken advantage of by a forward. It'd be like the farce that existed for a while with attackers kicking the ball at people's arms from point blank range and defenders standing with their arms behind their backs in response.
It's irrelevant what body parts you decide to be the crucial ones, as long as they're trying to measure it to such a degree of accuracy the current problems will continue whether they're comparing arms, feet, or whatever other body part. The whole thing needs a rethink as to how they're going to integrate it, same as with the Mahrez penalty the other day, the replay clearly shows there is contact between the two players so it makes it more difficult to ignore compared to a one of real time view from the referee where he could just claim he didn't notice. You're always likely to spot more infringements with countless slow motion replays but at the same time you lose the context and so whilst it exposes a lot of new incidents that are in need of a decision, a lot of the time it doesn't actually assist in reaching a convincing conclusion.
