The only reason they did six hearts was because they didn't have the space to put 15 more heart pieces in the game to make up for those 3 gained hearts. I like the idea of 3 hearts to make it challenging, but in the case of hero mode, it sounds like a need and when the enemies were doing a lot of damage even in normal mode, at the beginning.
Another reason 6 hearts is a good idea was because they put in a new function with the Wii Motion Plus, which you had to get used to, and gave you more health to get used to it, so there was less irritation that you "didn't do that" and ended up dying.