Pyrite,
The issue for me is that, unlike many positions, I believe that RB is one of the easiest to be able to come in and perform at.
Just this past year there were 4 running backs drafted in the 1st round. Bush, Maroney, Williams, and Addai. All four turned out to have a large impact on their teams. Some RBs from other rounds, including White, Drew, and Norwood, all had a decent impact as well. In fact, the only RB from the first 3 rounds who didn't was Brian Calhoun (drafted by Detroit).
In 2005 you had Ronnie Brown, Cedric Benson, Cadillac Williams, JJ Arrington, Eric Shelton, Frank Gore, Vernand Morency, in the first 3 rounds. All of these players had at least somewhat of an impect, although Shelton and Morency's contributions could be debated.
In 2004, Steven Jackson, Chris Perry, Kevin Jones, Tatum Bell, Julius Jones, Greg Jones, all of whom had an impact their rookie years.
My point to all of this is when you draft a guy in the first 3 rounds of the NFL draft, you aren't usually drafting part of a 3-headed RBBC monster. These players make serious impacts right away, and they are the type of player who is expected to start or come close to it.
Therefore, if we take a RB early, I do not expect the Bills to take 2 RBs in the draft.