The problem was that Yvain's comment doesn't admit the interpretation of referring to zero expected effect.

The fact that everyone else on the thread interpreted it that way shows that it does.

I could see that as being true if my reason for not voting was "Obama just doesn't inspire me that much," but what about in the originally mentioned case where my reason is bad weather? Do you think Democrats and Republicans are different enough that their algorithms for dealing with bad weather differ in a consistent way?

With a very big ceteris paribus in there somewhere. (The relevance Yvain being a Democrat is that we may expect other people with the same political affiliation to be more likely to also share voting technique. Apart from making that inference possible the similarity is not relevant.)

Yes, I realise this. But the difference between Republicans and Democrats is likely to be so small this is small consolation.

On the other hand, there is Democrats and Republicans arguable use different value systems.