I think much of the criticism against Berbatov is that he hasn't really provided that little bit of magic to unlock the defences on a consistent enough basis to warrant counting on it in such a tight situation, though.
Rooney can provide a bit of magic AND just muscle his way through a defence.
Hernandez is not just about pace, he's about clever movement, pulling defenders around and creating himself that bit of space. I can't actually remember too many goals from Hernandez where his blistering pace behind defences has led to a goal, to be honest. More often than not, it has been cute movement and anticipation.
I know what you're saying and we do have a nice mix of strikers with differing qualities. To me, Owen is a very similar type of striker to Hernandez, for example, which is probably why we have seen little of Owen this season (it hasn't been entirely down to Owen being injured - more Fergie wanting to give Hernandez as much game-time as possible). His experience aside, Owen has largely become superfluous to requirements now that Hernandez has proved himself more than capable and the "mix" is more important than the number of strikers.
Back in the day of Yorke, Cole, Sheringham and Solksjaer, I can't imagine a more diverse bunch of strikers but they complemented each other perfectly and, as I remember, we did quite well with them (
) because of the options they each provided Fergie with.
I have no objective reasoning for this but there are just some players who are brought off the bench and I just think, "Oh yes!" and others I just think, "Oh".