Excellent!
I think it's a matter of manners, I try to answer 100% of the comments that they make me, or failing that, leave a positive as a sign that I agree with the comment, since after all, someone has taken the trouble to read what I have written, and to comment on it.
I have also seen the problem you describe, sometimes I really want to have feedback with the writer of the publication, but they simply do not respond. It is very demotivating.
Yes, I don't always have something to respond with, so I often just leave an upvote in that case. Good point on how leaving a comment, not just once, but many times on different posts by an author, and they never respond, can certainly be demotivating to even bother.