I saw a decent argument for why BM's single target would be worth a damn in
this video if you obtained the new Legendary shoulders for BM in 7.1.5.
Basically the argument is the BM shoulders will let you bank your DBs to 2 stacks, so you can pop them both at 30-32 seconds left on BW CD, and then go into BW with two DBs up, which gives more potency to the artifact ability and makes the T19 4pc actually worth taking. I'm not sure if this would be BM over MM in terms of ST yet (as balancing has yet to be finished), but mechanics wise there might be potential.