What's that female elf doing in the movie, anyway?

I've had a number of discussions with friends about the role of Tauriel in Peter Jackson's Hobbit movies. I understand and sympathize with the objections that she was not given a more substantial role. The movie certainly fails the Bechtel Test (but so did the book). Some saw her as no more than a love interest. Others pointed out that it's hard to add a completely new character with real agency without changing the plot too much. She could have been the one to kill Azog, but then she'd be a repetition of Eowyn in The Lord of the Rings.

All these were intellectual arguments to me. On an emotional level, I adored Tauriel, although I could not articulate why. Then I read this interview with Evangeline Lilly, who played Tauriel, and bingo, yes, exactly.

She kind of, in my mind, becomes the voice of the audience. She speaks out what the audience is thinking and feeling, and that is such a satisfying thing, I know as an audience member when I’m watching a movie, I grab hold of that character because I need to hear it. I need them to say what is going on in my head. And Tauriel does that through the whole film.... I love that she says things like, “When did we allow evil to become stronger than us?”...
She's the Arwen of The Hobbit. Again, some folks are outraged one way or another about that character and the ways Jackson did or did not empower her. At her best, she is also the voice of the audience, our hope, our yearning for love as a redemptive power, our faith in the best part of people, our understanding of the choices of mortality and humanity. Tauriel does that for me, too.

Everyone's mileage varies.