I agree with your opinion, that's why I gave 2 merits to your post. hmm hmm.

From time to time I merit posts just because I agree with the user but I believe everybody does this occasionally. Yes it is not the right way to spend merits but I think it is fine as long as you don't overdo it on all one-liner shit posts just because they said "I agree with you".
If the post is successfully explaining the reasoning behind it and I happen to agree with that, I usually give merit(s).
If the post is giving only information, without any opinions, I again give merits. (having the OP's opinion would be a plus)
It is pretty hard to merit a post which you don't agree with however. I am not sure if I've ever done that before. (*edit, I remember now. I did it. But It is very rare.)