Nobody learned a lesson.
Skull Kid was an one-time villain in Majora's Mask. He isn't playable in Melee nor Brawl.
Zant was an one-time villain in Twilight Princess. He isn't playable in Brawl.
Midna was an one-time ally in Twilight Princess. She isn't playable in Brawl.
Tetra was a two-time ally in The Wind Waker and Phantom Hourglass. She isn't playable in Brawl... and Sakurai considered Toon Sheik instead of Tetra for crying out loud!
Vaati used to be a recurring villain in the Four Swords series and The Minish Cap but he is not relevant to the Zelda series now.
What makes Ghirahim any more special than any of these characters I just listed? Oh, that's right... he is more new than any of them... that's why he is more special.
I predict that by the time a new Zelda game comes out after Skyward Sword, people will be all "Ghirahim who?" and supporting whoever is the villain or the sidekick in that future Zelda game. People are already forgetting all about Midna now.