Dawnshine Wonder

While it is true that Discord's redemption arc should have been handled better, there have been times where his antics have been a useful asset in the show as well, after being betrayed by Tirek, he did cooperate with Mane Six in stopping him. Discord learning to respect Fluttershy is what changed him for the better, if that isn't a justification for Princess Celestia and her friends redeeming him, I cannot help you there. Even in the later seasons he did help defeat Queen Chrysalis, though his antics did put Mane 6 at risk, it was quite obvious he had no intention on betraying them at that time, he even remembered the time when Tirek lied to him. In my opinion, I think Discord's antics stem from his immaturity, because of his predisposition he treats situations as if they are a game.