Impact is great for solo play or PvP but for a group invironment they are a hassle. Also once you raid many mobs are immune to stun anyways so ...
I want to expand on this a little. Impact, as with any other random stun proc (mace spec, startfire) does no have a place in your spec if you wish to group alot. Stuns are something that should be planned in a group, not left up to chance. A poor timed stun will actually wipe the raid instead of save anyone. yes there will be times when you save the tank with a stun, but not often enough to be counted on.
Why? If you stun a mob, that mob is no longer attacking the tank. Tanks need to be attacked! Paladins build threat by blocking with their shield and the mob taking damage when they get hit. Druids & Warriors get rage built up when they get hit which they turn into threat. So, a poor placed stun at the begining of the fight will take the tank out of the fight as he will have a harder time regaining threat while not being hit.
Note: I love impact for solo play or non traditional (no tank) groups