I think berbatov plays more than rooney because its fergies highest ever paid player so he has to keep playing him and get him integrated into uniteds style of football. He is also the most mature in terms of age of the front members and at his peak age so thats another likely factor as to why he plays because he is proven in the premier league.
Another thing is, with berbatov even if he does not play well, he may still possibly get you a goal which is why he is on the pitch.
This all being said, Rooney is a better player and will play more/be less subbed imo as he gets older and if he can keep himself fit and continue to progress being he is still alot younger than berbatov.
That all being said, berbatov and tevez are of more similar levels in terms of quality because they can fit different situations. If hustling and running is needed upfront, tevez could possibly start ahead of berbatov and if goals are needed against a weak team, berbatov would probably start being the bigger goal threat imo at the moment.
Tevez is a very good player indeed but i dont think he is having a great season. He started very bright, but then went into a major dip, then found his form again but his awareness/decision making is still lacking at times (somestimes plays like milan baros with his head down or does not pick the right pass) and he needs to score more goals. He may have possibly lost some of his killer touch, i dont know. I still like tevez but i have the same complaints of him as i did last season. If anything, he has improved in areas but got weaker in others.
IMO...
Rooney is a better player than berbatov. Berbatov is a better player than tevez. But the difference between the 3 is not HUGE. All three are top players but rooney is the most rounded. Berbatov is possibly most collected. Tevez is the busiest.
I agree with fergie to pick rooney and berbatov as the starting pair too!
pz!