The more I think about it, I think the Niners took Coffee as an insurance back. They know what they have in Gore, but they also know he is good to miss 3 games a year. Before drafting Coffee we really didnt have a guy who could come in and back up Gore.
With Coffee having a very similar style, they can probably rely on him more in the future. Spiller would be the compliment to both those guys, and would give the Niners a backfield similar to Dallas and Miami. If Singletary wants the Niners to be a run-oriented team, then what better way to do that then to stock up on RB, OL, and defense. I think if Spiller is there, he gets picked, and then you wont see any more offensive skill players in this draft.