To me, it's clear why we play the 4-2-3-1. There's no real width in the squad, particularly with Petrov out and SWP not playing particularly well. So the only chance of getting behind the opposition back four is via the full-backs, Richards & Bridge.
Both are quite good at getting forward but Micah hasn't much of a clue what to do when he gets there, assuming he doesn't fall over the ball or his own feet. When it breaks down he's caught out of position but the idea of having two DM's is that one covers each wing, in theory and Elano will also cover the right back position as well. The central defenders should also be available to cover as well but Dunne has enough problems covering his own position.
Assuming Hughes gets who he wants in the summer, I think we will go 4-4-2 as a first choice formation and assuming Petrov & SWP occupy the wide positions, then the requirement for the full backs to get forward is reduced so we don't need two DM's.