What incentives do professors have to increase the PhD completion rate in their department? In other words:


  How do professors profit if a larger (rather than smaller) proportion of entering PhD students in their department complete their degree?


(At my department at least, it seems like the only benefit is "warm-glow altruism". A professor gains no more from having more students at her department complete their PhDs than I do by having fewer stray dogs in my town getting killed by traffic. A professor's career prospects does not seem in any way affected by whether 80% rather than just 50% of PhD students complete their degree. Not surprisingly, the completion rate, at least at my department, is closer to the latter.)

Note:  I am not asking whether it is a good idea to give professors stronger incentives to increase PhD completion rate. Neither am I asking for suggestions as to how PhD completion rates can be increased. Rather, I am asking what the incentives at present are (granting of course that this varies from place to place).
In the United States, the National Research Council (http://www.nap.edu/rdp/) regularly ranks university departments. One of the criteria by which departments (and thus universities) are ranked is the "Average of Median Time to Degree" with the presumption that lower is better. 

In my university, this results in the provost's office putting pressure on us to reduce our time to degree through a number of measures, some coercive and punitive. Most of the time, the coercion and punishment is directed towards the graduate students (i.e., after the sixth year they lose their rights to certain fellowships, they have to pay more for their gym and library memberships, etc.). 

But the department as a whole gets some attention. Our chair has been regularly sending out lists of our orphaned and missing ABD candidates (some now in their 12th year) and asking us to find and terminate them (either through graduating them, or formally asking them to leave the program). 

I've also been putting some pressure on my peers through the admissions system by suggesting (since they are my colleagues, I have no power) that people with many students on the books should refrain from taking on new students until they clear their backlog more a bit.

I should note that I'm in anthropology where: 1) students tend to go into the field and disappear for quite sometime; 2) doctoral students are mostly useless to faculty for slave labor as most of us work individually and not in labs (with the exception of some of our archaeological / biological colleagues).