The requirements for mentor system are fine. And you know what? 90% of the people talking about the mentor system won't be able to give decent advice anyway. A new player doesn't need to know a level 60 rotation, raid strategies, or how to tell just how awesome an endgame player you are.
New players need to know about the game systems that will help them play, and allow them to experience the game to its fullest extent. They need to know how to glamour gear, how to unlock dye and materia, where to find the different combat and crafting classes, and which classes they should think of leveling for which jobs. They need to know the best ways to acquire reasonable gear for leveling and dungeons (maybe they could use some donations from a level 60 player who can whip up a level 35 hq chestpiece in 5 seconds for less than 100 gil), they need to be informed about the existence of the housing wards, challenge log, and battle chocobos (Not only does the game fail to tell you these things exist, but they are unlocked by subquests that appear in a SEA of QUESTMARKERS. Overwhelmed new players often do not check that one vital questmarker in the Limsa Lomisa adventurer's guild, and miss out on the challenge log's 3-5 levels per week of free xp.)
And that's not even touching the concept of level 50 endgame, such as poetic tomes, hunts, and the long process of fulling unlocking level 50 roulette.
The requirements devs came up with are the best they could think of using in-game metrics, and you can tell what they were hoping players who fit the requirements will have learned about the game.
White mage is low on MP, and the bard could use MP song to help him out. But maybe the White Mage is about to pop Shroud of Saints, and recover suitable MP from that ability. Does the Bard know about Shroud of Saints? Does the Bard know what level Shroud of Saints is unlocked at? Should the Bard sacrifice 20% of his dps or not?
You've got two methods to learn about other classes' abilities: reading online, and trying them out for yourself by leveling a class. Guess which method can be proven by in-game metrics?