It seems to be a common opinion on Hacker News that you shouldn't write a comment just saying "I agree"; that's what the upvote arrow is for.
(I'm with you on not simply commenting "I agree", which is what I assume the gpp said. It would be good to add something to the discussion as well)
1) interesting comment that contributes to discussion with which I may or may not agree with.
2) I agree.
The first use case is preferable to the second one. If you happen to agree with a comment that isn't interesting nor contributes anything to the discussion, then maybe your agreement with it is irrelevant as well.