I've been encountering a lot of misinformation about taunts effect on threat. Granted, this is inspired by a pug where I happened to run into an entire raid full of the uninformed but I have encountered backwards thinking in good raid groups as well. Hopefully at least a few of the uninformed troll these forums.... hopefully.
The threat added by taunt is permanent! Taunt in itself does not add a set amount of threat so if you are #1 and you taunt you gain nothing but if you are the OT and are not #1 in threat you can taunt and it most certainly DOES add permanent threat when it makes you equal to the #1.
Archavon is the best example of this (and where I argued with an entire pug full of morons). If the MT is #1 on threat by a large margin the OT can taunt before the first time Arch picks one of them up and permanently gain an amount of threat equal to the difference between him and the MT. Then they are both way out in the lead and when Arch picks one of them up he immediately switches to the other tank rather than DPS. If you wait until he picks the MT up(erasing the MTS threat) then the OT has to quickly taunt Arch off DPS and is now only equal to the DPS, rather than way out in the lead. And he probably has little mana/rage because he hasn't been tanking/getting hit.
This works in all fights were the MT might get CC'd or just in case the MT dies. If there is an OT you want them to be #2 on threat before the MT dies, not trying to pick the boss up off the DPS after. OT taunts the boss, MT taunts him back. Boom, easy instant PERMANENT threat added to the OT.