Several recent posts have addressed whether the likely voter (LV) models are more accurate than the results based on registered voters (RV). My sense is that this is not a question that can be answered in general, but rather has to be considered separately for each polling organization.
Let's look, for example, at Brian Schaffner's recent post, where he compared the RV vs. LV results for several pollsters in the 2004 election. He found that "On average, the RV samples for these eight polls predicted a .875 Bush advantage while the LV samples predicted a 2.25 advantage for Bush, remarkably close to the actual result." He concludes that "it does appear as though likely voters did a better job of predicting the result in 2004 than registered voters."
This quick look at the data is hardly conclusive, of course, which Brian acknowledges. He had only eight polling organizations in his analysis, and despite the average results, four of the eight polls showed no advantage to using the LV model, while the other four did. As he suggests, it would be important to look at other years, but also other types of elections and other polling organizations.
Even then, however, an overall conclusion would not be especially helpful. Each polling organization's LV model is so different from another's that each organization has to look at its own success rate over the years to determine whether the LV model is helpful. In 1996, Gallup's senior editor, Lydia Saad, showed that in some presidential elections, Gallup's LV and RV results showed little differences, but when they did differ significantly, the LV results were more accurate in predicting the election results. That was at the national level. In the New Hampshire primary Gallup poll results over time, however, it's the RV results that typically were marginally closer to the final election results.
Still, at the national level, I would always bet on Gallup's LV results being a better estimate than the RV results, an example that Brian found for 2004. Gallup's final RV results showed Kerry up by two percentage points, while the LV results showed Bush winning by two points. Pew, which uses the Gallup LV model, also showed a four-point swing, from a one-point Kerry victory to a 3-point Bush victory.
Other polling organizations, of course, could find different results. And trying to average the results across polling organizations, to determine whether "in principle" LV models should be used, I would argue, is not helpful. That question has to be addressed by each polling organization based on its experience with its own LV model.