Windfury and Vessina offer additional reach, which allows fewer minions to do more damage. This is particularly advantageous in matchups where your board is constantly at the risk of getting cleared. When these cards are timed correctly, they can win the game on the spot. Windfury, in particular, is a risky tech because it is terrible in the Aggro mirror. It does nothing in the early turns and if the Even Shaman falls behind in the mid-game, the card becomes a brick in hand because the Even Shaman will never be able to attack with a minion worth putting windfury on. Vessina, on the other hand, is much easier to play into Aggro because it is a minion. This alone puts it way above Windfury.