you can totally cheese a fight with 6 heals/2 tanks though if there are no dps checks. The fight will go on longer, which means more mechanics, but it's not like those mechanics will be just slightly easier, they'll be completely trivialized. Especially since you have 6 resses and no time limit. Almost everyone could die and you could easily recover. Every single fight in the game would just become a battle of attrition. It would absolutely not make the game richer. It would make it silly.
Edit: Also, @ you earlier post, vanilla WoW totally had dps checks. Less of them than now, but they were absolutely there.