You don't know he isn't AP though.. No matter what player anyone prefers could end up being a bust, we may love the pick then in 2 years be complaining it was a horrid idea, that's just how it works.
As for CJ Spiller is a different back type imo and Richardson I can't say much I didn't follow him that draft but just from youtube clips how he turned out doesn't shock me but I am watching video with hindsight so that skews it of course.
Granted better chance he isn't AP than is but we don't know either way. That's aside I do like Miller and perfectly comfortable him being our starter.