We used to get direct data that DID come from Google, called PR, and people STILL called it inadequate in reflecting what it was using to measure the value of sites. It didn't tell us exactly how G was determining page rank, or how it tied in to relevance for ranking purposes, so we still had to speculate the rest. Google is never gonna tell us everything, folks, so we will have to use the best third party info from DA, DR, TF, AS and such to give us a clue.
These metrics do in fact help us estimate ranking power, as page and url authority definitely is an element of that. But link authority is not the whole picture, as is often pointed out. The problem is the metric is over extended by its users, who think it covers contextual relevancy and traffic as well. When not misapplied to those aspects, DR et al is very helpful indeed in building the total ranking power of pages and domains. Don't throw away the bricks, just because it isn't the piping.