I think Pogba starting in a deeper role when we're in possession works well. When he's too far up the pitch, teams press him quickly in numbers and stifle him. In the deeper position, he has the extra time and space to pick a pass. If the other team stands off him, he can move quickly forward with the ball into a more dangerous position. If they send only one player to press him, he has the ability to get pass that player. If they send more forward, they risk losing their defensive shape and open spaces for our other players.
The thing about Pogba is he has so many tools and options from that deeper position, all of which can test the other teams defence in different ways. That's priceless.
He does not need to learn to be a better defender, that's not what we bought him for. Playing deeper doesn't automatically mean more defensive responsibiities. Sure, he's more than capable of helping out when needed, but it's Herrera and the other midfielder who need to do that more. I for one think that Herrera is doing a terrific job. We are just one versatile midfielder short of a truly world class midfield trio, and I don't see us needing a Carrick-type player anymore.