January seems to be that time of the year that I ponder this question often. After the grades have come out, I always get a few students with very low grades (D or lower) coming to talk to me, and wondering whether this grade will have an impact on their future ambitions of going to a top graduate school and becoming a professor.
Another related event that happens in January is that it's the interview season for tenure-track positions in my field. The market is getting increasingly tougher, and plenty of very strong postdocs that I know don't have any interviews.
Watching these things always makes me wonder if it's a bad thing to be discouraging to the students.
Sure, technically it's true that even with a couple of failing grades, you can still gain entrance to a top graduate school, given glowing recommendation letters. And sure, I'm sure that some researchers weren't phenomenal in graduate school but they flourish in their later career.
However, in today's tough market I think that these things are becoming rarer, and in general, unless you stay on top of the track you don't really succeed in becoming a professor/researcher. So I often wonder if it's better to tell the students a bit more of the honest truth. That their ambitions are possible to realize, but that the competition is tougher than they could ever imagine. That they should definitely have a plan B in place. That even if they succeed in becoming a professor, their life might be very different from how they imagined it to be (struggling with a two-body problem, working for a university that you never dreamed of, and so on). I, for one, might have appreciated an honest answer, and maybe that would have changed my career trajectory. And I think enough people with doctorates work for jobs that they could have gotten before going to graduate schools.
So, as an educator, it is bad form to be discouraging. But I wonder if it's actually in their interests to really know what's in store. Do you do this? That is, do you tell your weak students with unrealistic goals that their goals are unachievable?