They have Villa for the center and it doesn't really make a difference if Pedro plays on the left or on the right. It's not like they made all those rotations just to accommodate Pedro in the team.
To me it looks more like they made all the rotations just so Messi would play in the center and they are looking for players who can play on the right (Sanchez, Rossi) just so they could keep Messi in the center.
their current formation is a function of the payers they currently have, that's all. - Villa-Messi-Pedro works better for them than Villa-Pedro-Messi ... that doesn't mean it would be the same if they had Aguero, though.
Also, with Barca one cannot just restrict players to certain zones of the field considering their 3 attacking players, especially Messi, move so freely all over the field, making them even harder to mark and close down.
This whole Messi on the right or center stuff is just for the starting line-ups lists ... in reallity, once the game starts, Messi has as free a role as anyone in football.