Disagree, it's not a singing contest, we have plenty of shows for that purpose alone (Idol, X-factor, The Voice), we don't need another one.
However, yes, if a song is badly sung, it deserves to fail.
But, a well sung BAD song deserves to fail aswell, because in the end of the day we're looking for the best songs and not voices.
I think Ell & Nikki's win was unfair for plenty of other reasons than their singing to be honest.
Your comment regarding Eli & Nikki is spot on. My first reaction was: "This? This is the best song of the entire contest? THIS? Am I on Punk'd?" Plus I find universally favorable comments of commentators kinda... weird. Now, I'm not insinuating anything, but all along it was like "why bother watching, it seems that it has been already decided..."
Now, back to Pastora's song... I pointed out few days ago in a thread dedicated to Humpty Dumpty Engelbert that this is not a radio contest and that great voice simply doesn't cut it anymore. And it is true, it takes oh so much more than that... However, with such a good interpretation (I dare to say goosebump-producing climax), this one just cannot flop. And it's not only the voice, Pastora actually carried out the entire song the best way possible. You just trust her when she sings, you don't even have to know a word of Spanish because of her gift to transfer the raw emotion to the audience.
After all, let's remember Marija Šerifović. Of course, she has amazing voice, but it was her quirkiness and odd looks that landed her the trophy that year, not only voice. She captured the audience without fancy stage gimmicks and choreography. All it took was her voice and her vibe. And I kinda sense that Pastora will do great this year.