Because that is defining in terms of how you support a candidate and are likely to vote for them again. That's quite different from an ideological term that is not given any definition. Navy Pride in many cases displayed liberal ideas yet pretends to be a Conservative. What use is asking what people are when they support policies contrary to what they say they believe?Think about approval polls - whether the president is doing a "good job" is entirely subject to a person's definition of a good job. Nevertheless, approval ratings are very good proxies for support for a candidate.
I suppose so, but the lack of definition in the key term is real questionable.Furthermore, your point doesn't address the fact that even if the poll is useless alone, there's nothing to indicate that it's not useful in determining trends.