Vivenna at the Safe House
Vivenna is right about what happens to a person when they lose their Breath. It is a part of your soul, and without one, you are more prone to depression, you get sick much more easily, and you're generally more irritable.
I included this mention here because I'm betting that most people who read the book side with Denth and assume he's right when he talks about these things. But don't be too judgmental about the Idrians—yes, they're biased, but the Hallandren are too in a lot of ways. It's not as simple as one side always being right and the other wrong. In this case, the Idrian teachings are correct, and most Hallandren are looking for justifications when they say that giving up one's Breath isn't all that damaging to them.