Posted this in the 3-5-2 thread but it didn't get much of an answer and I'm genuinely interested to hear what people would think of it so I'm going to try here:
Ancelotti proved last season that there's nothing wrong with an asymmetrical formation if it lets you use every player in the position where they can do their best work for the team. Here you sacrifice a wing-back in order to get Januzaj on the pitch in his best position, as a winger/wide AM/inside forward (whatever you want to call that free role from a wide starting position). His defensive work-rate is decent, but really it's down to Fletcher and Jones to provide the energy and defensive nous to cover that vacated area.
This way, you can get Januzaj into the starting line-up (and Welbeck would also be a superb fit there). You add pace and unpredictability into the attack, and also strengthen the threat from the wide areas. You add in a type of player that has been central to Van Gaal's formations since Alkmaar - an unpredictable, pacy wide attacker. He used El Hamdaoui at AZ, and Robben at Bayern and for the Netherlands. Januzaj fits that role like a glove.
Yes, it's
slightly more defensively open. But it's not as if Ashley Young, the current alternative, is exactly a defensive guarantee as a wingback. Jones is already stepping up into those areas regularly, and Fletcher is the sort of experienced midfielder who can more than cope with covering a little more of the field defensively. The 3-man back line allows for exactly this sort of adjustment, because if Jones has to step out, he still leaves an intact pair of CBs to cover the central areas.