Harmony - Troop Symmetry

As I understand it, the idea wasn't just to add suppressions, but to replace the magic schools that aren't studied by these races with them. Then the chance of getting several unnecessary skills will remain the same.

Not studying is one thing, and being rare is another. Even if a school isn't represented in a guild above level 3, it's still more useful. But taking away the Humans' native Darkness would essentially deprive them of a powerful and in-demand weapon. If Darkness is preserved for Humans, then a 2% suppression on non-core schools could still be acceptable. Without increasing the chances of this absurdity.

Added after 1 hour 25 minutes

5. Humans learn Light, suppress Darkness, Chaos and Summoning are not their thing, but they will still have two magic schools in the guild, so they can learn Summoning in the guild, and therefore can also learn Summoning and suppress Chaos.

Summoning doesn't fit Humans; Light and Darkness are what make up the nature of Humans and are reflected in the gameplay. Summoning is a school for magical races, implying high magic and knowledge, spellcasting, and education. Again, no one is forcing you to play a dark knight; the choice is yours. But unlike Suppression or Summoning, it's impossible to call Darkness useless for Humans.

Interestingly, would you like it if Humans were deprived of Light? Well, that would be quite realistic - besides, Humans get more benefit from Darkness - teleport, mass haste, and stone skin can also be obtained from skills, but the debuffs from Darkness cannot be replaced with anything.