I think we're getting somewhere now. The OP's point is a valid one. It certainly helps you create a more realistic picture than hearing Ray Rice is like Emmit Smith. Rice is not going to finish his career as the leading rusher in NFL history.
I'll make one more attempt at this before I give up. Not sure if any of the rookie

guys / Draftniks want to hear it, but here goes.First, the comparisons to Emmitt Smith or Tomlinson or other good/great backs are not about their projections or upside for me - and I would hope everyone gets that. No one should be saying that Ray Rice WILL BE the next Emmitt. That's ludicrous. What can be said, though, is that when you watch his style and his collegiate highlights, he reminds me of Emmitt Smith, Frank Gore, MJD - whomever. That's the comparison.
Now, as for discussing ceilings and floors. Ceilings are worth discussing but floors are not. Here's why:
Ceilings tells me what a player CANNOT do. If someone tells me that Player X in the best possible system (offensive scheme, great blocking, good coach, 20-25 touches, etc.) will produce like this guy - then that's a good thing to know. If he lands in what I think would be an ideal spot, now I know what his upside is. Now, if I hear that he lacks breakaway speed or good receiving skills or he will struggle to pick up a complicated offense or pass blocks or whatever, then that's worth knowing. In other words, I know what he won't do - get 3,000 all-purpose yards or 30 TDs rushing - but knowing he has the potential to be a Top 5 or Top 10 fantasy RB is worth knowing.
Floors tell me nothing - sorry. Guess what - the floor is zero. Despite all of what you say, I guarantee that one of your expected floors will be too high. I'd hazard to guess that 3 years ago, no one would have put Michael Turner's production for the past 3 years near where it is today. Sure you can say he has a starting job today, but that doesn't help me over the past 3 seasons. Let someone else drive that new car off the lot and have it sit in their driveway for 3 years.
Further, no one would have put Ryan Leaf's floor where it wound up. Nor any other first day draft pick bust. You just don't do that - but it happens.
So that's why ceilings are worth discussing and floors aren't. I already know the worst case scenario (injury / never play), but I don't know everyone's upside. If those two come together next weekend, I want to be ready to draft accordingly.