I think that the motivation is partly to allow for people who don't care, or otherwise have no opinion. But also, responsible pollmakers need to allow for the possibility that the other options they offer are incomplete. Or otherwise unsatisfactory, for one reason or another. Any responsible poll needs to offer a "none of the above" option, and everyone who responds to that option shouanld be added to the error rate for that poll. "None of the above" ultimately means "my views are not adequately represented by the options presented by this poll, and I do not wish to be counted among any of the groups this poll enumerates." For those who chose this option, we don't really know what their actual opinions were. We just know they thought the options presented were insufficient. Thus, the only remaining option (after the poll has already been conducted) is to include all "none of the above" repsondents as part of the overall error rate of the poll. We don't have any idea what those people thought. That doesn't mean we should ignore them - at the very least, we should pay attention to how many "unknowns" there are, and commission better polls if the number becomes too high.