Seath the Scaleless is Better at Lore in Dark Souls than at Combat
This results in a monster that could have many spectacular build-ups, depending on the quest lines the character pursues but it doesn't make sense in the way that other encounters that are based on lore and lore can.
In addition, with Seath the Scaleless as an one of four bosses who will be given the Lord's Soul, Seath is one of the primary targets for players to track for on their way to taking over Gwyn.
Beyond that initial betrayal Seath's story continues through two major events. He ultimately is named Duke and left an elixir of a Lord's Soul. Additionally, he has a daughter of a crossbreed named Priscilla
This backstory is well beyond other main villains, Gravelord Nito or the Four Kings, who have their place in the lore , but not as significant in the entire series.
The crystal will never repair itself or becomes active after hitting it one time to damage it. This is something even Seath himself can do through accident.
When the crystal is cracked, Seath isn't much of threat so long as the players are able to discern which from his 3 attacks is he performing while they're within the attack zone.
It can also be topped off with the previously mentioned tail cut that transforms the battle from being boringly simple to a gruellingly challenging, since Seath's tail lies distant from him, and he is able to spin around at the speed of a mockingbird.
Thus that locking Dark Souls famous Moonlight Greatsword behind this added annoyance means the majority of players won't have the chance to use the weapon in the game.