It's in the campaign's interests to paint is a tight race, because they don't want to promote political apathy. They want people to vote.
To put it another way if his model suggested, e.g., Obama has a 100% chance of winning, and everyone knew that, his supporters would be less inclined to vote and he could lose. So there's a chaotic element to these things that's just difficult to predict, and journalists are better placed to handle this chaotic element (another example: Hurricane Sandy). That said, given the abundance of polling data, I think his model stands to be a much better predictor than journalistic intuition.
Depends how "alternative" those candidates are -- it might convince more people that they can vote for their actual favorite instead of issuing a strategic vote for whichever of the two major-party candidates they find less objectionable than the other.
To put it another way if his model suggested, e.g., Obama has a 100% chance of winning, and everyone knew that, his supporters would be less inclined to vote and he could lose. So there's a chaotic element to these things that's just difficult to predict, and journalists are better placed to handle this chaotic element (another example: Hurricane Sandy). That said, given the abundance of polling data, I think his model stands to be a much better predictor than journalistic intuition.