It's very objective. I wouldn't say that a high grimoire score means much. It would mostly depend on what you're specifically checking what someone is good at, and then checking their grimoire progress for that particular thing. Mine is 4650, I'm pretty sure, which is decent as far as overall score, but I'm a pretty average pvp player, but I have the grimoire for a lot of the pvp categories. I've done plenty of raids, but I don't have near as many completions as a lot of people, but again, I have the grimoire. It's all objective. The only time I could see grimoire being checked for skill is the number of kills in pvp or raid completions, but even for raid completions, it doesn't necessarily mean you're good at the raid. TL:DR - pvp grimoire is the only thing you could use to accurately judge someone's skill.