I'm not a Gabriel Yared fan. I'm really not. And that's why I haven't bought this album (nor, for that matter, do I plan to). But I feel obliged to point out that the score is effective in the film. A more theme-oriented approach may have worked better, but Yared's minimalist style does not at all detract from the film.
Also, it seemed to me that the bluegrass and Yared's underscore blended rather well in the film (as to the album I have no idea). Stylistically there was really no problem.
Though one must wonder about Yared's themeless underscore--can you imagine a John Williams (The Patriot) approach to the film? A love theme, a main theme, a war theme, a travel theme, a theme for Kidman, one for Zellweger, and one for Law--somehow I don't know if that kind of thing would have been that effective. The plot of the movie meanders all over the place as Kidman and Law live out their lives, slowly finding their way to each other--perhaps Yared's "meandering" strings are appropriate for this?
I don't really know. Maybe if I get a hold of the album I will be able to better communicate what I'm trying to say.