In Jeff Angus' ongoing series on The Seductions (& Giant Sucking Sounds) of Metrics: The Average is Not the Territory, he talks about how it is easy to get distracted by The One Great Number (TOGN).
Tip # 44: Don't be fooled. The seduction of a TOGN is understandable. And TOGN schemes have some value. But the TOGN is not the person; ignore that at your own risk.
The problem with any metric is that they provide too convenient a hook for people and, as Angus indicates, can obscure the real story. When looking at averages, you completely miss what is happening behind the average. Is there lots of variation or only a little? Does the variation have a pattern or is it (apparently) random? When looking at trends of composite numbers, do you really understand what that number represents? What is the context behind the composite? Does it encompass everything that you need to know about the process it represents? Not likely.