I would agree as a general principle but in practice, if a team is looking for one type, why draft the other at all?I think a guy that can contribute ~ 75+ catches a season in the passing game can certainly be drafted earlier than a pure rushing RB. Kind of like drafting a pass rushing DT in the first is more favorable than drafting a run stopping one...
A player's value is somewhat scheme/ philosophy specific. That's the conundrum GMs are forced to face. You want a player, but will he be there?
There are so many moving parts on draft weekend, flexibility in plan is key.
I don't mind taking a guy a half round "early", if Flo/Grier have a strong conviction. The way the league seemingly looks at the RB position though, I see very few scenarios where taking one RD would be necessary.