Grimoire shows how much you are invested in the game and is generally related to your skill. KD doesn't matter as much directly with skill because someone could have used to suck at crucible but got better and started getting more kills. If you have over 3k Grimoire then I'll assume you are a decent player unless proven otherwise (lol I played trials vs. a group with 3500 Grimoire+ each and they all sucked)
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The best players are the ones who have killed 5000 thralls and dregs. The ones who havent all suck.