I have also been one of the people who think pirlo should sit this one out but really, when you think about it, there are too many scenarios where he will be called for. I know the reasons for why he shouldn't be included: his lack of speed, his 1st leg sloppy passing, his lack of defensive pressure (although did well last game), but here are some reasons why him starting is a good thing for us.
He'll be able to help us maintain possession. When he's in the side, yeah we pass the ball around more but we also keep it away from players like Ronaldo. I think that's a huge factor in being able to beat real.
He'll be able to control the pace of the game. If real have numbers up front, he can do a lot of damage when in space. I've seen two players successfully be able to hit the opponents defence with a lobbed one touch through pass, Pirlo and Marchisio.
He has the most CL experience out of anyone in the side. He's been there, several times. That's why the pressure doesn't get to him as much and yes this is a double edged sword. As long as he keeps his dribbling in defence to a minimum, we won't suffer. The lack of pressure he feels can be an advantage however, when it comes to dead ball situations. When he's called upon to end the game with a free kick, there's no one in the world is rather take it than him.
It's a tough choice and there's an obvious tradeoff by playing him, but I think people shouldn't be too quick to want to exclude him. Let's not forget how good this guy can be. Hopefully with Pogba back, we will have more options, and since Sturaro was great last time, our midfield in Pirlo, Marchisio, Vidal, Pogba, Pereyra, Sturaro is a luxury to choose from.
Also,I have a question. When was the last time we played with Pirlo, Pogba, Vidal, Marchisio?