Like Fur said, it's all about speed break points.

At some points in time in TBC, you could go backwards in DPS due to global cooldowns and spell rotations. If you search EJ now somewhere in there is buried is talk about how they changed the way it works so you'll never have a negative dps gain from adding haste now.

You'll still want nice planned out rotations for longer fights since you'll have a good dps gain their vs the random spell casting without the bonus damage. Trash pulls shouldn't need that kind of attention, otherwise they're not really the speed bumps that they should be.