I cant comment about heroes in regular Multiplayer skirmish - this is something we`ll decide on during the beta.
As far as Campaign goes - half of the gameplay is around the heroes.
In MMORTS heroes are essential - every one of your armies needs a hero to lead it, and only your main hero can accept quests and travel to the quests locations.
MMORTS heroes are tough, one can take out about 2 battalions of regular infantry on level 1, but as someone has mentioned - that`s not enough in the game with 1000s of units. To compensate for it we are providing heroes with variety of powerful special abilities.
But it looks like that might still not be enougth. If so - it looks like we`ll have no choice but to go against our storyline and turn heroes into battle-mages, allowing heroes to learn magic.