Well, Deidara defeated Gaara but only after losing his left arm, and Gaara kept protecting the villagers from ´D´s bombs. What would have happened If Gaara could have focused on Deidara without worrying about the villagers.
Kakuzu fought Yamato, Kakashi, Narutom, Ino and Choji. Kakuzu almost captured Naruto after N failed to land the blow, If they hadnt interfered... would Kakuzu had injured Naruto badly and killed the rest of em?
And Pain DID defeat Naruto, If Naruto hadnt transformed into the fox.... he would have been drained down to unconsciousness and taken away if it wasnt for Hinata. Pain NEEDED Naruto alive so he did hold back.