I don't buy the argument that Urthemiel's soul needs to be preserved because it is a unique part of the world and it seems a waste to let it to be destroyed.
By that logic...
1) Why does Morrigan approve of the Warden defiling the Urn of Sacred Ashes. It is also a unique relic and it seems a waste to let it be perverted.
2) Why should we kill Flemeth ? If we strictly stick to Origins, Flemeth is a powerful witch capable of shapeshifting into a High Dragon and a large bird. She is also a unique part of the world. Yeah, she might want Morrigan's body for possession but surely if Morrigan is dedicate to preserving that which is unique and powerful, it should be a small price to pay.
There is also the obvious fact that Morrigan never explains the purpose of preserving Urthemiel's soul in clear detail as well as the reality that you are going to potentially bind and shackle an innocent child to a destiny that he never chose, which is to carry the soul.
Now some of you might say that we should do the ritual anyway because it will save your life ? Newsflash - The Warden is a dead man/woman walking. They are already serving a death sentence and they will die being turned into a ghoul or possibly worse. True, we learn in the Trespasser DLC that the Warden might have found a way to stop the Calling but that's DAI. In DAO, all you know is that a Warden will die a miserable death in the Deep Roads. So why not just go out by killing Urthemiel ?
Does this mean that you have to absolutely sacrifice your Warden. Of course not. You can let Alistair or Loghain do the killing blow instead. However, it is important to remember the fact that if you know how you are going to die and you know it will be very unpleasant, you won't see Heroic self sacrifice as a bad thing.