The purpose of comments
- To get feedback on what you've posted
- To know readers are in sync with your content
- extension of your article.
When the 3 parameters are met definitely the writer might judiciously give an upvote.
As some people have commented above me, that when you arrived at the post you see a lot of comments you will be discourage to comment because there is this feeling that the writer will not get read you comment so sometime i read take what is needful and leave without a comment. This as been a relative concern to you @kevinwong. My own take is not to reply to comment so soon, like after 6 to 12 hours you start checking comments, replying, (the reply should be like what the commenter will have to reply back), with this you will know those that just come drop comment and zoom off or those who are really in touch with article.
For spammy comments, it is your shot to call