The brawl is not representative of a realistic scenario. The hunter deck is extremely weak and slow, which allows you to get away with doing low-tempo crap like spamming your hero power.
Against a real deck, you're going to be playing an opponent that likely spends all their mana every turn (and not on stupid crap like lock and load followed by half a dozen traps or pinning shot), against whom you can't use your hero power or you will simply lose unless you're lucky enough to draw the perfect answer to their minions by turn 4. Even as a midrange or control paladin, an extremely strong board-control deck that already massively benefits from spamming its hero power, you almost never want to use your hero power until turns 7+. Thing is, using your hero power is a massive tempo loss relative to spending 2 mana on a card, and if you've used your hero power 4-6 times while your opponent spent that 8-12 mana on playing actual cards instead, you've probably already lost the game even if you get that discount giant, that's been sitting in your hand all game leading up to that point, being a dead card instead of something you could actually play to win the board back before it was too late.
Problem with frost giants, as with all giants, is that they're conditional, and when their conditions aren't met they're dead cards in your hand; they may as well have a card text that says "when drawn, discard this card" on them. What makes frost giants extra bad is that you actively have to set up the condition that makes them playable, giving up tempo to spam your hero power. Y'know, you don't give up tempo to play sea giant, nor do you have to pay mana to lose health to be able to play moltens, and you don't run mountain giants unless you're playing a deck that already relies on holding tons of answers to make them cheap enough to play.
But who wants to spam their hero power? Maybe you want to spam it in the mid-lategame when you have some inspire card out and maybe have played your justicar, but that's way too late to start enabling your frost giant that's been sitting in your hand all game as a dead card costing 10 mana because you haven't had the time to use your hero power so far since you've needed to play actual cards to keep yourself from dying to the enemy's board.
Uh, if all you have to play is one card anyway then it doesn't matter what it costs? I mean, unless it costs 9+, which this wouldn't in the arena during a topdeck war anyway.