The difference between Polymorph and Polymorph Boar is insubstantial when Fallen Hero is in play, I agree. But there are several key things to keep in mind here.
1.) The Hunter deck has crap for early minions and its only early removal is arcane shot. Further, the Hunter suffers from a complete lack of board clear ability aside from UTH which a slower deck really needs. That means your Fallen Heroes stick around for quite awhile and it is very easy for the mage to establish a board presence and stack their inspire effects. So compared to Constructed, where a Turn 2 Knife Juggler basically dies every time, you can throw down Fallen Heroes anytime you want and they're probably going to stick around.
2.) If Fallen Hero is not in play, you're looking at a 6-7 mana removal combo, which is worse than the comparable 5-6 mana removal play you'd see from regular polymorph.
3.) The Mage deck can just get away with some truly awful shit because of how bad the hunter deck is.I was playing hunter against a mage and they played a minion and IMMEDIATELY turned it into a boar to attack my face, and they weren't even threatening lethal. I laughed when they did it because they literally just spent 2 cards and 5 mana to summon a huffer. You couldn't get away with that shit in constructed.