Do National Polls Mean Anything?

"National polls don't mean anything at this point," as we all know. Chris Bowers, however, notes that they may not mean anything at all:

By sampling between 40-50% of all registered voters, the vast majority of national Democratic primary polls are not specifically sampling the Democratic primary electorate. As such, these polls should not only be taken with a grain of salt, but should almost be dismissed entirely as useful indicators of the current state of play in the Democratic primary / caucus season. If they are not polling the Democratic primary / caucus electorate, or even coming close to doing so, then they should not be used as indicators of opinion in the Democratic primary / caucus electorate. To use an analogous example, one does not poll the entire nation as a means of determining public opinion in California. One does not poll all Democratic self-identifiers and leaners in order to determine the opinion of Democratic primary and caucus goers.

This seems correct. We know from history that the universe of Democratic primary voters is different from the universe of Democratic self-identifiers. The problem, from the pollsters' perspective, is that with the primary calendar constantly shifting around, it's far from clear that beyond the traditional early primary states one would have anything on which to base a turnout model. Who's likely to go to the Nevada caucuses? Who knows? Of course, from the media's point of view this is all irrelevant since accurate forecasting of elections and accurate portrayal of public opinion aren't things media outlets are interested in. Shocking, yes. But they're trying to attract attention -- i.e., readers and viewers and ad dollars -- while minimizing costs. Actual candidates, however, do have an interest in accurate information and it would be fascinating to know how they handle this.