It seems like you are specifically asking about the word "POD" right? As opposed to "I agree" or sometimes, "This."
Is POD essentially different than those two or any other way of acknowledging agreement? In my mind, it's not. Those are all the same way of saying the same thing.
And, as has been said above, I seldom see (and probably never use) them with no further comment. I don't think it's bad etiquette to just say "POD" or "I agree". I assume the purpose of such an abbreviated post is to just sort of cast one more vote in favor of the cited opinion.
I usually don't just stop there because, well, I always seem to have more to say!!