Am I wrong or taunt tooltip clearly says that it has no effect if u already got Agro?
Furthermore if u ever loose Agro using taunt on cd (and that will happen on some encounters), ur fellow group member will probably die, cos u will not be able to pull mobs off him.
Neural Dart Tooltip:
Fires a neural dart that taunts the target to attack for 6 seconds. Player targets deal 30% less damage when attacking anyone other than you. Effect lasts 6 seconds.
The 110%/130% threat increase isn't in the tooltip, but no where does it say that it has no effect if you already have aggro. Also, having your taunt on cd is not necessarily a bad thing as it means you are stepping your threat up by increasingly large margins upon use. Someone might say "I wait for them to pull aggro before I taunt," but by preemptively taunting you can bypass them pulling aggro.
One question this brings up that I had not considered before: If I am tanking and the boss is on me, the boss will not switch targets until another dps reaches 130% of my threat level. Say they are at 120% of your threat level at some point and you use a taunt. Does your threat jump to 130% of your current threat level or go up to 130% of that dps that is at 120% of your threat, despite the fact that the boss is still attacking you?