Spiller probably sat because he's a rookie facing a huge rush. Rookie RBs generally can't pick up a blitz or a rushing linebacker to save their lives, and so my guess is they sat him for more experienced backs that had a chance of blocking. If that is the case I understand the reasoning, but I think maybe he should have seen the field a bit more.
The other problem is that if you read the board here, fans are in no mood to be patient with a rookie RB. He's going to make mistakes, he's going to miss blocks, and he's going to dance around before hitting a hole because it worked for him in college. Like the rest of the team while the Bills are rebuilding, it will take time for him. But it seems as if idiot fans don't realize that, and are impatient.
The fact is fans MUST be patient with a new coach, new schemes, new GM, etc. As bad as the Bills have drafted over the past 10 years, we're going to have to go through this before it gets better. If you are a kitty and can't hack it, I suggest you stop watching for a year or two.