When going to a comment page, I often want to express my approval for that comment by upvoting that particular pro/con (and maybe Liking the comment if a feature for that gets implemented).
For example, Mandrill quietly pulled the 12,000 free emails plan within the last few days. http://www.slant.co/topics/136/viewpoints/1/sections/15/comments belongs to a con that could use a bunch more upvotes.
Rather than up voting I would use like buttons, like here in discourse. This basically means no down voting.
I see what you mean, @Cawas, and that's a neat idea too - liking comments
What I was suggesting was to have the up/down arrows visible for the pro/con the comment belongs to.