It might look like a suggestion for a change, however I'm pretty sure it is unlikely to ever happen so I'd just like to see your thoughts on this.
Why to give it to tanks? - well its all-know fact thats with Cata Blizzard brought in the idea of bring the player not the class so they gave hero to Mages and Hunters, however it's not impossible one of the classes to be lacking from the set-up, but there will always be a tank and a healer, so If either tanks/healers had Heroism instead of the 3 dps classes that have it now it would be better imo. I might be biased playing a tank myself and that's why I propose tanks but not healers, however it also suits the idea behind the tank better, yea fearless reckless frontline warriors(or paladins, druids, DKs ;pp), that at point can fanaticize his party, to turn the tide of a battle!