I'm pretty sure that it is Atlas's punishment to hold up the world, not his job.

Yes I agree the destruction of Piett and his crew was rather cheesy in Return of the Jedi.  You spend the last movie giving this ship a sense of menace yet all you have to do is hit those big things on top to lose the deflector shields and kamikaze the bridge with the smallest fighter in the Rebel fleet.  However, some of the later Star Wars novels implied when the Emperor was killed by Darth Vader, the Imperial forces lost the will to fight because the Emperor had a mental control over them.  Without their leader, Piett and the fleet went into utter panic and chaos.  The rebels got the advantage and destroyed the Imperial fleet.

You have to remember that the emperor had been killed and he had been controlling everyone, so that helps a little.