Pique is a defender who is good in the air, distributes the ball well BUT he is as slow as Kewell's stepovers and has been skinned many times by any forward with half-decent pace. In the break-neck pace of the EPL, Fergie must surely have felt that Pique could not cope. Remember, Fergie even sold Stam when he thought that the Dutchman had lost a yard or two of pace after his injury.
Furthermore, his relative inexperience could cost United points especially since he is playing in a crucial position (CB) rather than fullback where mistakes are not so costly. I've seen Pique being pulled in all sorts of positions by opponents when he started for United, that left Rio isolated in the box.
This move (if it happens) would probably be for the better of both parties.
1. Pique wont have to feel disappointed and kick his heels on the bench again while waiting for an opportunity to break into a solid defensive axis of Rio-Vidic (or Brown).
2. Pique's style of play is probably more suited to the Spanish Liga where the pace is not too searing. In all redpect, I think he will be a big star for Barca in the League but not in Europe.
3. The low transfer fee is firstly because Pique is not English, secondly because Fergie does not want to make it difficult for his player to leave. Fergie sets low transfer fees for players whom he has a persoanl affiliaiotn to and so would not try to scupper their transfer and chances of first team football by setting ridiculously high fees that might turn clubs away. This has been the case with Nicky Butt (5-7 million only for an English international and glittering trophy cabinet), Phil Neville (only 3 million, must be Everton's bragain signing apart from cahill) and Tom Howard.
All in all, I think this proposed move is a win-win situation for both Pique and United.