Im not saying Bale could not do a job, sure he would probably be good and be successful, but bales main strength is his top speed and having space to run into.
The amount of posession barca have like against chelsea, bale wont ever get a chance to do that.
Alves is alot more tricky in his play, he plays 1/2's and has a very good eye for a pass, when he gets behind the defence the majority of the times he picks a player out.
Plus on the other side is they could perhaps become too unbalanced; Usually when Alves attacks Abidal would sit back and help cover, with the 2 of them attacking could they become too exposed?.
I just think rather than spending 40m on a british player who might never work in there system, they can buy a local lad for a quarter of that price that knows there style of play and country inside out.