How faculty are expected to allocate their time differs somewhat across settings (research universities, liberal arts colleges, community colleges, professional schools), but all professors struggle to achieve the right balance between teaching, research, and service.
This cartoon captures the essence of the problem:
http://www.phdcomics.com/comics.php?f=1060
Cal Psych Ph.D. Navigator 
Granted it’s just a comic, but a bit telling that “mentoring” is not included in the equation. The best advisors I’ve had in my life were able to balance all four (mentoring, teaching, research, and service) to varying degrees.
Great point. Mentoring typically gets lumped in with ‘teaching’, for the sake of simplicity, but the skills required are only partially overlapping.
In my experience, mentoring effectiveness is judged in terms of tangible outcomes: specifically, the number of students a faculty member has trained who have been placed in academic positions. This criterion is problematic, since a) it perpetuates the notion that the only worthwhile path for a Ph.D. candidate is a faculty position, and b) at least in theory, it creates an incentive for faculty to encourage undecided students to stay in academia. How ironic, given that good mentoring means figuring out how best to support a student in pursuit of her/his own goals!
Our department is making an effort to recognize and reward good mentoring, both at the graduate and undergraduate levels. But, as I’ve argued elsewhere on this blog (see the category titled “Relationship with your advisor”), every student needs to be proactive about cultivating the mentoring relationships that s/he needs.