As far as I remember, Roose Bolton is not important in any way until the second book, except as a name attached to one of Robb's generals.
Uhm, he ties up Tywin, which enables Robb to take out Jamie. And he does it commanding the North's main force, which he retreats in good order, rather than the completely inaccurate random sacrifice of 2000 men that we had in the show. He might really come into his own 2nd book, but if you don't introduce him in the 1st his side flipping is out of context and doesn't mean much to the viewers. It's like not introducing Joffrey until he cuts off Ned's head.
His progression made perfect sense to me when I read the books, but seemed to have nothing at all to do with that battle.
In book 1, he's just a named general. I know he's an interesting character and all later on, but that is all he was when I read book 1. "Hey, let's give Bolton the command" says one character, in the way names are dropped to show just how fleshed out the world is, and the other character says "Good idea". That is it. There is no careful plotting from Roose Bolton during the early battles. And, in fact, he is introduced as an actual character in book 2, and does not seem like
he is going to turn sides until book 3, when it is most advantageous to do so. In fact, Martin carefully makes sure that nobody suspects how opportunistic Roose is by making his bastard look like a rogue agent.
I don't think he had any lines in the first book, either, so casting him in season one is actually a bad move. When Bolton does show up, you want him to have the right actor. He's likely difficult to play. The good actors aren't going to hang around as an extra for a year.
I am worried that this whole "Tyrion getting knocked out" thing is going to lead to skimping on all the battle scenes in future events, though. They get a bigger budget, sure, but how can they afford to make Blackwater what it is in the book? They would be foolish to not show a battle at all, because the Battle of Blackwater is
filled with strong images (Sansa looking out over the Bay being one). The problem is, it would cost a fortune. They could try changing enough elements to cut down on the whole thing, but it's going to send ripples through the entire season because Tyrion spends almost all his time prepping for it.