while usability testing is perhaps no more reliable a prioritization method than an expert-level, qualitative evaluation performed by a lone reviewer or a small group of reviewers, testing is like any other evaluation or discovery method: It must be, but frequently is not, put in context. Page views and time-spent-per-page metrics, while often foolishly considered standard measures of site effectiveness, are meaningless until they are considered in context of the goals of the pages being visited.
Is a user who visits a series of pages doing so because the task flow is effective, or because he can't find the content he seeks? Are users spending a lot of time on a page because they're engaged, or because they're stuck? While NYTimes.com surely hopes readers will stay on a page long enough to read an article in full or scan all its headlines, Google's goal is for users to find what they need and leave a search results page as quickly as possible. A lengthy time-spent metric on NYTimes.com could indicate a high-quality or high-value article. For Google's search workflow, it could indicate a team's utter failure.