The most logical reason to focus on "time to degree" for doctoral students is that most of them say they want to finish -- and most graduate departments say the same thing. People are happier and programs are more efficient.
SAN DIEGO -- In theory, these days, everyone agrees that attrition in Ph.D. programs is a real problem. Graduate students don't want to spend years in programs from which they will never graduate, and universities don't want to support those who won't complete their programs. Also in theory these days, most academics agree that it's crucial to expand the diversity of the Ph.D. pipeline so that the candidates for faculty positions represent a broader demographic than the current professoriate.