This may be a good move or not. What I mean is that Pirlo was once a hell of a player on both defending and creating. Right now at 32, he can't run as he did. But the class is still there. I think that defensively he isn't the player that he was, but in creating he is still one of the best.
Currently we play with one pure defensive midfielder (Melo) and one midfielder that plays further on the pitch (Aquilani), who should mark, should create, but he doesn't need to run like Melo does. As we have seen, Aquilani can't mark as his position demands, he is good at the other 2 tasks. Not exceptional but good.
So, if we sign Pirlo in order to get rid of Melo and play with Pirlo and Aquilani next year, then we are screwed. Pirlo at this age can't cover half of the distance that Melo usually does. If thats the case, the problems with Aquilani are still there, and we will have a worse midfield pair next year with Pirlo instead of Melo.
On the other hand, if we sign Pirlo in order to get rid of Aquilani, I think it could be a very good move. We will have still Melo to do all the dirty job and additionally getting forward whenever possible and at those times Pirlo will be able to defend. Additionally, Pirlo can mark better than Aquilani, he is better at creating, he is experienced enough to be able not to get completely anonymous in some matches and he does not need to run a lot as Melo will do most of the dirty job. If this is the case, I like it. I like having a midfield pair consisting of one energetic and one very calm player, both of which can pass well, create, mark and also have a nice long shot.
So, in my opinion it really depends on what we are signing Pirlo for...