Is it ever ethical to skimp on comments?

I've found myself pondering this question today. Is it ever appropriate to give few or no comments on a student paper? I don't mean for a particular assignment and treating everyone the same. What I'm wondering is whether it's ever appropriate to give extensive comments on most of the papers in a given batch but less on others.

I find myself wondering this because I'm pretty sure I just marked a paper where I spent more time reading, interpreting (a monumental task at times), and making comments on a paper than I think the student did. In this case, this student has also attended only 3 of the 7 weeks of term. The paper shows it too because we talked about each of the elements that were supposed to be present in the paper over the last 7 weeks and most of them were absent from this paper.

I know I shouldn't take it personally and I don't when students decide not to attend class. But I do find myself resenting the time (and brain power) I spent trying to figure out what this student was saying and then trying to provide constructive advice to perform better. I felt like I was investing more in this than the student was. And I was seriously tempted to just slap a letter grade on it and skip any comments.

In the end I did write comments. I hope the student takes them to heart. But somehow I doubt it. I don't feel any better for it though.