I can see the point of inverted wingers when you have people like Bale and even Lamela who can check inside and curl a great goal-bound shot.
At a stretch I suppose I can see a case to be made for an occasionally inverted wing back who can surprise a defence and ghost in at the far post.
Doherty does neither of those two things.
To say he does nothing is actually an overstatement, he actively hinders forward play on the left as his only option is to turn back and pass back to Winks (last night) or who ever it is, killing all momentum stone dead. As Harrison says above, it must be the easiest thing in the world to defend against.