What are you talking about ?
Pirlo is not a defensive liability, Pogba in that position is. Pirlo might have lost a ball or two last season, as the years are catching up and he isnt quick enough on those turns anymore, but that hardly costed us two goals. On the other hand, Pirlo is in the top 3 distance run every single game he plays. Picking out opponent passing and running lines, pressuring the build up play in his area, he does quite significant work.
Pogba on the other hand, whilst beeing far ahead for his age, lacks the positional skills to be effective that deep. He doesnt see how he has to move when in possession to create passing lanes like pirlo. He doesnt position like pirlo/marchisio when not in possession to lock down passing routes.
Ball controll, is mostly valuable, when under high pressure of opponent. Its a massive asset for box to box midfielders, wich pogba certainly is made for. He has the long pass to start counterattacks when receiving the pass from deep, has the presence to go into duels all over the field, has a dangerous shot once close to the box, and the technical skill and ability to skip past a defender for a shot, or exchange passes with the strikers
When facing an opponent that parks the bus, THEN its good to play him the deepest of the 3 CM's. Simply because he has the best passing range compared to vidal/marchisio. Because he has a longer shooting range and better distribution in general. And because those other two are more lethal close to the box.