The entire point of hiding comment scores is to allow people to judge for themselves how much a comment is worth – your expression of agreement is ruining that somewhat.
Whether you can see points or not, I would think "I upvoted" comments with little content would still get downvoted.
Although, one side effect might be: When you can see a comment is highly scored, a reader may think, "everybody gets this guy, no need to help him argue the point. upvote" Whereas when you can see a comment is lowly scored, a reader may think, "this is a good argument, I'll help carry it, upvote and comment". But when you can't see scores, making the call to upvote and comment, or just upvote is made only on the content of the comment. Whether that's good or bad, I'm not sure.
