The general rule states that the pieces are stronger, especially at the beginning and in the middle game, but this is not always the case.
If the rooks have open files and can create direct attack against the enemy king they might be the better pieces.
Have a look at the following example:
Short,Nigel D (2440) - Tempone,Marcelo (2335)
Wch U20 Mexico, 1981
White's activity is overwhelming and it is no wonder that he can start decisive attack.
16.Bxf7+! Rxf7 17.Nxf7 Kxf7 18.Rae1!
The last reserves are coming in the action and Black cannot hold the king's flank.
18...Bf8 [Or: 18...Bd6 19.Bxf6 gxf6 (There is an in-between check in the line- 19...Qxf6 20.Ne4 Qxf3 21.Nxd6+) 20.Qh5+ Kg7 21.Ne4 where Black is helpless. Just have a look at his flanks and you will know why.]
19.Ne4 Ra5 [The rooks are ruling in the line- 19...Nbd5 20.Nxf6 Nxf6 21.Bxf6 Qxf6 (21...gxf6 22.Qh5+ Kg8 23.Rg3+) 22.Qh5+ Kg8 23.Rf3
While 19...Bg4 loses material- 20.Bxf6 Bxf3 21.Bxd8]
20.Nxf6 gxf6 21.Re8 Qd6 22.Qh5+ Kg8
23.R1e7! Qxe7 24.Rxe7 Bxe7 25.Qe8+ [Black resigned because of the line-]
[25.Qe8+ Bf8 26.Bh6 Nd7 27.Qxc8]