To me its about whether Guarin can really play behind the strikers. If Conte sees him as an ideal player for that position, then I guess I am ok with it.
If on the hand, he is just a sub for Pogba, Marchisio etc then I think we are truly overkilling the central midfield area in the squad unless Marotta has already conceded the fact that we will lose Vidal or Pogba during the summer window.
Let us not forget that Vucinic's market value is still around 10m. That values Guarin at 15m.
Assuming he uses Guarin as the trequartista in a 4312, then I can see this happening in Europe:
Starting XI (4312): Buffon, Lichsteiner, Barzagli, Bonucci, Chiellini, Vidal, Pirlo, Pogba, Guarin, Tevez, Llorente.
Bench: Storari, Caceres, Ogbonna, Marchisio, Asamoah, Giovinco, Quagliarella