barcelona already have over 70 percent posession most of the time, changing to 3 at the back simply for the sake of having an extra midfielder doesn't make sense as you'll be sacrificing the marauding wing backs which are a key part of the barca style, ok it worked great against villareal but if you play that narrow every week teams will catch on and pack the centre of the pitch with midfielders, and it will be difficult to break through no matter how good your players are. If pedro, villa are the only wide players in your team then opponents will simply not bother with wingers and put 5 centre mids in the middle, if they play a lone striker you will have 3 at the back marking one player which doesn't make sense either, the current 4-6-0 is perfect for barca, there is no reason to change it. The reason for the change against villareal was because of missing personel, and made sense having 3 at the back against 2 strikers, but it won't be the permanent formation for barca.