While I don't "Earn" hard ROI on voting comments and trying to keep the vote above dust. I do gain commenters and spread some good will and success to others. In addition by keeping my blog a great place to comment and have a discussion, I also find others that are active and engaging on the site.

Personally, I feel it is still worth upvoting comments and engagement and I do believe it adds value to my account in something I would call "Soft ROI".

It's not tangible but I think my blog does better as a direct result.

Just offering another point of view.