having Pirlo on the field can be very important if Allegri decides to leave the ball to them and play on the counter. Pirlo has played some great long passes forward last few games in the run to Morata and others, they can become a dangerous tool while having all 3 of Morata, Tevez and Pereyra upfront, with Vidal too getting into the box and waiting for a return ball. defensively, Pirlos positioning is still decent, with the help of the other 3 mids he can close down spaces against a team that isnt used to having possesion and controlling the game. Pirlo can become a liability defensively only when we are holding possesion and the opposition press him. when you limit his time on the ball and make him play passes forward as soon as he gets the ball, then hell do fine. also, while having Pirlo on the field, we are creating more space for Marchisio when transforming from defense to offense, since the opponent will probably try to cover Pirlo mainly to prevent quick passes forward. Marchisio has excellent passing ability, he made quite a few long-ball assists and great long passes that didnt end in goals this season + there is Bonucci who can be that "plan C" when Marchisio and Pirlo arent free to receive a pass. having two guys (+ Leo) capable of delivering that quick long ball forward can help improving our counter attacks a lot. i would however not play Pirlo in case Llorente will start, we play too slow when both are on the field.
anyway, depending on the gameplan Allegri wants to use, playing Pirlo is ofcourse not a terrible idea. the guy has appeared in numerous important knock out games, both for club and country. i like Sturaro, the guy can run the whole game and cover a lot of ground, but he is not Pirlo. he is not the guy that can decide the matchup with one FK, long shot or killer pass. if Pirlo will play (which will happen most likely) then i can totally understand what Allegri had in mind.
one more thing. people here think they can see what a liabilty Pirlo is defensively and Allegri somehow not and that he is scared of the press to bench him

lol

which is ridiculous. Allegri is perfectly aware of what he gains and what he loses by playing Pirlo. if he puts him in the lineup, its because he and his staff decided it is best to do so, not because hes scared of what GDS or CDS or other garbage papers might say.
im sure the Maestro is gonna shut up his critics once again