Honestly though, why do national polls matter at this point. We have a much more accurate poll already- the actual votes of half of GOP primary voters. And trump is up roughly 10.
They matter because 1) National support isn't fixed. Different candidates could be gaining or losing ground, and 2) Most of those votes were cast when the field was larger, so we need new polling to show us how the race has changed now that some of the candidates have dropped out. Ideally, that would be achieved via polling of all the states that haven't voted yet, but realistically we're not going to get new polls of all ~18 or so remaining states every week, so a national poll is a decent proxy.
Let me put it this way: Suppose that shortly after March 15, we got several national polls that showed Cruz rapidly closing the gap on Trump by picking up the bulk of Rubio's support now that he'd dropped out. Wouldn't we consider that significant information, because it likely reflects a dynamic that's occurring in the ~35% of the country that hasn't voted yet, just as it is in the portion of the country that's already voted? Since the actual national polls that we have don't really show that happening, isn't that also significant information?
