You can be winning the whole game, but then still lose, and vise versa.
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The reason I like KP so much is that it can be used defensively as well as offensively. I use it to buy myself an extra turn when I'm digging for a late game Tidal Wave etc, and that turn can sometimes be the difference between a win and a loss. It also gives my allies a bit more survivability when I'm struggling for field position. I could always go with something like Rain Delay or Holy Shield, but those cards both have their own weaknesses and they can never be used offensively.
All in all I love having KP in the deck. It is a play-skill card, which puts the responsibility on me to sac/play/hold it at all the right times, and at this stage that's something I really want to hammer into my brain. Remember, I won't be using this deck forever but I will be using the skills I develop in these early stages of my learning throughout my SE playing career.
KP is good for humans because they allies generally lack in attack department.
For shadow, for example, 8/7 Brutal Minotaur is a bit of overkill. But with humans you can get 4/2 Kris with KP and Aldon out, which is hasted 4 damage for 1 cc, and you can retreat it and cast again, not to mention what happens when your opponent has DoU out.