It's a good point. I'm basing my pov more on how I view backup rb's in general. It's easier to rank backup wr's. There is more skill in that position.
With rb's, there is a top level. And those guys are pretty clear. McCoy is one of those.
After that, there just isn't as much differentiation. How many times have we seen a backup come in and run wild for a few games? Most are fast, most are strong. Again - putting a 2nd round tender on a 6 carry guy is kind of silly, and something no one here was calling for until the specter of the mighty pats showed up.