Simply put I don't think there's another position in football that is so dependent on other people doing their jobs to have success than a running back. Look at the top RBs in the league today - Johnson, Adrian Peterson, etc...they have phenomenal offensive lines in front of them. I think there's value to a running back but if I'm starting a complete team (which is what this exercise is about) I'm not spending such a high pick on a running back. The bust rate and flameout potential is simply too high.
Then again, that's my view on things. If 22407 decides to draft three OL in a row, Johnson's value could be even higher. I just think without it, you're not getting the value expected of the #6 pick.
FTR, I would not have selected Johnson as the first running back taken.