Simple Beam can be used to either get rid of a good ability on the opponent or to trap the opponent, or can be used to change a bad ability on one of your Pokemon (Slaking and Archeops, anyone?).
Also could be used to speed the boosting of a set up sweeper(note that if you intend to eliminate your foes ability they might anticipate that and switch to something that would benefit from simple(if eliminating abilities gastro acid is a better choice)
Lastly, I'm pretty sure abilities such as Traunt, Slow Start or Defeatest have tricks preventing easy ability loss (blocks ability replacing moves <Simple Beam and worry seed will fail> though Skill Swap, Gastro Acid and Mummy all still work)