United still revert back to a 2 man midfield in the easier games they play, which is a difference. The other teams you mentioned set up similarly regardless of who they're playing. Look at our league games this year;
Started with 4-5-1:
Chelsea H - necessary
Bolton A - Rest Rooney before Munich tie, no Owen or Macheda available
Liverpool H - necessary
Wolves A - Rest Rooney before Milan tie, no Owen or Macheda available
Villa A - necessary
Arsenal A - necessary
Birmingham A - a bit of an odd one, Berbatov wasn't available, and Ferguson didn't like to use Rooney and Owen together? Even Diouf came on ahead of Owen. Didn't pay off anyway, 3 shots on target all game.
Started with 4-4-2:
Fulham H
West Ham H
Everton A - lost that 3-1, and we were even up against an Everton midfield 3. Beat them comfortably 3-0 with a 4-4-2 at home though, probably thought it would be enough to get by
Portsmouth H
Hull H
Burnley H
I would guess we'd have played 4-4-2 against Wolves, Bolton and possibly even Birmingham if we'd had the right players available for it, and that for the most part shows that we'll still play 4-4-2 when at all possible. We've even needed to stick three in midfield against a two man Bayern midfield at home!
You could argue that these remaining 4-4-2 games are some of the last we'll see, and you could be right, and in that case I don't think Berbatov will be useful enough for us to keep. But I really can't see us setting up in a 4-5-1 against lower table teams who are likely to combat us by packing everyone into their defence, and these are the kinds of games where Berbatov can give us an extra dimension. Like I said earlier in the thread, these are the games where Chelsea, Liverpool and Arsenal have dropped points time and again, and this season we've been far more efficient in them.