The reasoning behind the Mk3 is to remove the interference from the ringer circuit that happens after the master socket.
May find that other FTTC providers have been switching over without getting Openreach in, so BT are doing the same.
I don't see why they don't supply you with a plug in master socket adapter plate filter as standard instead of the microfilters.
Come to think about it they make money selling them at the BT shop.