I think you hit the nail on the head when you say that we only have a limited number of votes to dole out.
I feel like the correct etiquette is to vote on what moves you. If you have a hit piece you're not going to be able to vote up everyone.
If you're the commenter, you shouldn't be commenting only for rewards, but because the article was valuable to you and made you want to engage. if your comment doesn't get votes, so what. If it was insightful, people will remember and associate you with insightful commentary.
You'll build your reputation for being a quality commenter and in the long run that will be more rewarding. This article moved me because I was thinking about this last night, so have an upvote! No reciprocity needed, unless you want to!
Well said! Comments should add to the overall community engagement and in turn, authors should recognize good replys.
Hence your upvote! :)