What incentives does an advisor have to help PhD students graduate in a timely manner? I've heard it suggested several times on this site that advisors (especially in experimental fields where the PI may benefit from a cheap, highly skilled workforce) have an incentive to prevent their PhD students from graduating in a timely manner, or at least that they have no incentive to actively promote graduation when the student is "ready." (Whatever "ready" means for that particular student.)

For example, user47148 says


  You are the only one who cares if you finish. To your advisor you are cheap labor.


bfoste01 says


  I often wonder if keeping highly-skilled cheap labor around to do something with the data that a PI is collecting from a high-profile grant is often an implicit factor that impacts students' trajectories. In which case it is not uncommon to see students in their 6th or 7th year of the program receive funding.


and grandmah77 refers to


  keeping your advisor from keeping you there forever as cheap labor. 


I am interested in learning more about the opposite: what incentives do advisors have to help their students graduate as soon as they're ready?

This question discusses incentives for advisors to increase their PhD completion rate. Presumably this is part of the answer, because a student who isn't allowed to graduate in a timely manner might just drop out instead.

But besides for that, what incentives (if any) does an advisor have to help students graduate when they are ready, rather than dragging out the length of their degree?

(In some programs, there are policies that limit the duration of PhD funding. I'm asking about programs where PhD students can potentially hang on for several years past the mean time-to-degree.)

P.S. answers supported by references to actual data would be amazing.
This is very simple. I care about my students, and want the best for them. Languishing in graduate school is rarely optimal for anyone. And then there is the issue of funding. For the same reason -- concern for my students' wellbeing -- it is very important to me to provide them with RA funding as close to year-around as possible. If funding starts to get tight, it doesn't help to have someone taking a luxury sixth year when he or she could have graduated earlier. I'd rather have everyone funded on RAships than have so-called "cheap labor" from senior students while junior students are forced to TA. 

If the question is referring to why even a sociopathic advisor would want his or her students to graduate on time, there is some prestige in having students graduate quickly and move on to good positions, and some shame in having students in their sixth, seventh, and later years.