He's shown a higher top level at RB than any of our other options, and I would say, anyone in the PL outside Ivanovic. He was progressing steadily and had a great 2012/13 season, his injuries came back next year and became worse this year, and it didn't help that his coaches didn't trust him.
Darmian is untested and I would trust Rafael to be a better backup and better competition than Valencia (we can always play Young there if they do actually get injured together)
Again, as a Barca executive, would you have kicked out the 19 year old Messi simply for being injury-prone? A young Iniesta for not delivering on his potential? Xavi for stagnating and playing second fiddle to Deco? Pique for not making the United team?
I'm not saying Rafael is comparable to any of these players; I'm saying he has shown enough to be kept ahead of Valencia. Unfortunately, I know it is not going to happen.