Because we have little mobility in the centre of the pitch. So by playing wingers on the 'wrong wing' it means both wingers should come inside. Doing so crowds the middle of the pitch a little more and then if we lose the ball we at least have bodies in there capable of winning it back.
The alternative is to leave Barry and De Jong in the middle and let Johnson and Bellamy hog the touchline. Since De jong drops deep, it leaves Barry with 2 or 3 men to mark/play against.
So in a nutshell i think thats why he does it. In an ideal world, just as Johnson or Bellamy cut inside, the full back should overlap. Nedum didn't really do this, nor did Bridge (to any effect). To play this system you need two quality full backs. Bobby seems to have signed one already (Boateng)so maybe he'll go for an equally athletic left back.