The aggro get added to the person who got hit. however in fights like saurfang and rotface the dps shouldnt really be getting hit. saurfang adds should be left for the ranged to take down. if the melee are cleaving saurfang, they are doing it wrong and need to hold off using cleave abilities until the adds start moving towards the ranged. as for rotface, the little slimes arent really an issue, they do minimal damge, and cannot be taunted, so any ret aura proc on them isnt a worry. As for the big slime, if ur dps are getting hti by him, they will most likely be killed, and/or ur add tank needs to do better.
as for the threat component, rets dont really need to use ret aura. it does minimal damage/threat, and there is usually a better option for auras to be used; frost aura on sindragosa, or using shadow resist if u dont have a priest. etc
even if ur ret pally uses a diff aura, the raid still gets the damage/haste bonuses, as they are applied just an aura being uses. they are not tied directly to ret aura.
lastly, if ur warrior tank cant get aggro back from a dps affected by ret aura... yer... u might want to have a sit down with him and have a chat.. even direct him here