I agree with the first post on this topic for a number of reasons.
1. If someone has said everything you were going to say, It makes more sense to simply indicate agreement than restate the post.
2. There are a number of posts that are extremely relevant to discussions, but get lost in the realm of random small comments. I think that adding a system letting people promote relevant posts would help improve the discussion on the forum.
3. People will do the current I agree posts simply because there is no better way to support someone's thread.