He is the meat in a Smalling - Shaw sandwich, he basically has nothing to do.
To an extent I actually agree with you and I'm fully in favour of him playing there.
The way I look at it is that we play a system that is in essence 3 defenders at the back. The way we set up to accommodate that is Darmian naturally covering inside for Smalling who operates as our only genuine centre back and covers Blind's space. Blind has the permission to carry the ball out and also the responsibility to cover Shaw who has license to press forward.
So as a unit we cover from right to left with Darmian, Smalling and then generally speaking either Blind or Shaw in the back line. The reason we can do this confidently is because of the cover provided by our 6, be it Schneiderlin or Carrick.
As a unit it's excellent and works very well and would be appreciated more if the front 4 were operating as well, with more dynamism and actually putting away chances.