I think that's the point -- "very sad" is unpoetical because it doesn't convey anything more than "sad", which is itself a generic word. If you're writing poetry or fiction you want to be describing emotional states more interesting than "sad".

The important corollary here is that not all language needs to be poetical. It's OK to use generic language in a lot of situations. Sometimes "I'm very sorry to hear that ____" is the tool for the social situation.