Well let me start i don't comment anyone for the upvote. I do however appreciate the upvote whenever i get one. It means the author of the original piece either enjoyed my comment or appreciates the time i took to read it.

Then to answer your question.
I myself am plankton. As you can see in my wallet everything i earn goes back into raising my voteing power. At the moment i only upvote for 0.02$. What benefit could you have of upvoting me? I would come back to read more. On the other hand if i don't like a post i don't comment it. But i am shure alot of the steemit users do. So in the end you would make more money in the long run. But if you would vote on yourself you would be getting even more.

What i believe is the best insentive is that people keep using steemit if they get a great upvote once in a while. Meaning users who would normaly quit after a week now keep on commenting and creating great content, as a comment or as a blog post. So what you are douing by upvoting is keeping the community active and present on steem. And lets be honest that is the best reason to upvote people. To interact and keep them busy and happy on steem.