Being magical and wondrous is not correlated to being worth preserving. The spirit of Wisdom from All New, Faded For Her is magical and wondrous, and it is worth preserving. The dragon cult of Haven is magical and wondrous, and the world is better off without them. We don't directly see enough of what Urthemiel is to know which category to put him under, but that he apparently led his high priest into the Black City and so created the Architect seems like it might be a hint.
The Wardens know their Ritual works. It has four times before. The Warden is basically taking Morrigan's word that this will, and we don't have the option of asking her how she knows it will work. It'd be one thing if Morrigan could demonstrate that Flemeth was possessed by Mythal: that would give you some reason to think someone involved in this knew what she was talking about. As it is, the Warden has some room to wonder just where Morrigan and Flemeth are getting all of this.
Yes, we do, later in the series. It's a piece of soul that belongs to an Evanuris. Very likely to Flemeth herself.
She's trying to purify a bit of her soul that was stored inside a high dragon and that was corrupted by the taint. No wonder she risked sending the most precious thing to her, her own daughter, Morrigan, to accomplish such task.
That's why Flemeth helps the Wardens. It's in her interest to help you get to the Archdemon, even at the risk of seeing the creature slain by them and the bit of soul inside it lost forever.
If you don't think saving Flemeth's soul is worth it, it's within your right as a player. But you do know Flemeth never did anything to harm your character -QUITE the contrary -, much less does the DR have ANY negative consequences to the Wardens. For the game to call it a Dark Ritual is plain stupid, since it's a magic rite made to conceive a life and purify a soul. Its name should be Purification Ritual, Ritual of Renewal or something similar. Therefore, accusing Morrigan of deceiving you forsome sordid selfish reason of her own is unfair. She's not an evil witch trying to take advantage of you. She's simply a mage born of a very powerful ancient mage who has found a way to both help save the world from a great evil (the fifth Archdemon) - at a smaller cost (spare the Wardens of the Ultimate Sacrifice) and to solve her problem of recovering a fragment of her soul.
Plus, we also know the taint might've been created in Arlathan, since its capital has become the Black City, where the Magisters became infected with it for the first time. No wonder Flemeth knows a great deal about it and came up with the Dark Ritual. She KNOWS how the taint works, UNLIKE the Wardens, who obviously no longer have no idea what the taint is and are desperatedly grasping at straws to deal with a magical enemy they know next to nothing about. And if that includes making insane demands such as forcing people to drink infected blood, become infertile live only up to 30 years and give up their lives, so be it.
That's all there is to it. The player can choose to make an informed choice or stay on the path of ignorance by following the Wardens' tradition out of a misplaced sense of honour.
Guess what would be the right choice if the Wardens of today knew anything about the taint at all...