Exactly, and correlated errors, where a polling error in one state predicts similar errors across the board.
I disagree that it's all pointless though. Most basically it's smart for campaigns to have a good model and let that inform strategy where appropriate. Since the president is a big deal other people's decisions are also impacted, and in the long run it pays to have good predictions of those chances. Also, the outcome sometimes is fairly certain and that isn't always easy to see.
I agree, it's far from pointless. The 538 model is arguably close to the best you can do considering how difficult the task is, but it's important to understand it as purely a reflection of the polling data (and 538's reliability scores for polls), and that polling data is inherently flawed. After all, there are only 2 ways to perform a perfectly accurate poll: either know the outcome a priori, or run the election. We shouldn't be too surprised when models like 538 fail to correctly predict the outcome, because that's not what they represent. It's an analytical tool for understanding the current state of polling.
I disagree that it's all pointless though. Most basically it's smart for campaigns to have a good model and let that inform strategy where appropriate. Since the president is a big deal other people's decisions are also impacted, and in the long run it pays to have good predictions of those chances. Also, the outcome sometimes is fairly certain and that isn't always easy to see.