We just don't have the personnel for it -- not the wingers (not even close), not the centre mids (Fell, not to mention Rodwell, are too positionally naive and Arteta's attacking suffers horribly when he's trying to help cover for one of them), and definitely not the strikers (you do need two, good healthy ones with a lot of games together or at least experience playing 2 upfront, and too often, we don't even have one).
And our defence ain't exactly the solid rock bastard it once was. And even then, it usually had a dedicated DM protecting it.
I'm all for changing up the formation to freshen up our attack and keep opponents guessing. But for us to consistently play the 4-4-2 would probably be disastrous.