Hearthpwn

I would add that, unlike your secrets, you can trigger this spell's effect during your turn. That is a really big point that we can't undervalue.

It's because the Thrall card has a lot of downsides. Jaina is 9 mana for a minion, armor, extra effect all game, AND a more powerful version of her hero power. Thrall gets armor, no minion, a far more situational battlecry that works once, and a completely different hero power.

He cannot make totems anymore, which can be really bad for shaman and this power. What do you evolve if your foe can keep killing your stuff? More problematic, what if your foe has been clearing stuff since the game started and you have nothing to evolve with the battle cry? Furthermore, evolve has high risks. I have a 4/4 taunt, and then I evolve it into a 7/1 that can't attack ... so sad. This isn't to say evolve can't also help you get really lucky, but anything with luck is typically considered worse than a surefire answer. (There is a reason why Yogg is only used as a last resort by many pro players.)

The 5 mana cost then is because of the drawbacks of this card and, if I had to make a guess, also because (token) shamans typically are designed to just win at the point this card would most likely want to see play. I don't want to evolve one minion on the 20th turn of the game, I wanted to do it on turn 5 when all my minions hadn't been flamestriked. Another way to look at this is that this card is a way for shaman to have another 5 drop like bloodlust so they can reliably end the game before their foe can remove the board.