Remember, all of the points that you make – and you make good ones – regarding the value of upvoting content that you like applies equally to you and comments that you find reassuring, valuable, or just interesting. As you said, it costs you nothing to upvote things except the opportunity cost of voting something up and not another thing.
One of the things I am very aggressive about is making sure that people who comment on my work with something meaningful, I engage with. I upvote their comments, even if it's just a little bit. I make sure to reply to as many interesting and useful comments as possible. I ignore (rather than down vote) comments which are obviously just trying to game the system, because I have more, positive content that I want to reward that each and every down vote would take away from that.
Cultivating a positive, engaged community around your work is one of the best things you can do. Even if you think of it just as a way to earn more, not just for your art but for being a part of the community, for letting people touch you and be touched by you, you'll find it has positive results.
Every little bit helps, and that goes both ways.
You said it better.
I absolutely agree with you, this is why even tho' I am busy with work and personal stuff I take the time to read every single comment. Some of them are from people that just read a single sentence from the post and just drop a random message regarding to that without understanding the whole context. Or they just read the title and say some unrelated stuff. Such as the case of the following message that you can find few lines under: