His left foot and stamina and work rate are what we need most urgently. Right now our left flank is still very weak, with only Chivu, Nagatomo, Zanetti and Mbaye as options at LB, and no LW in sight. So we will play with Cassano/Sneijder/Coutinho cutting in from the left, onto their right foot, but meaning we won't get many quality crosses into the box from that flank. Now that Maicon has declined seriously (more mental than physical, but just as impossible to reverse the fact) we need players to pick up that slack. Hopefully Nagatomo and Jonathan are up to the challenge.
But the important thing is that we add someone left-footed, who is full of running. The fact that Pereira can play as LB or LM just makes him that much more valuable to this Inter.