The very low quality flag for questions is described as:
This question has severe formatting or content problems. This question is unlikely to be salvageable through editing, and might need to be removed.
A question that is primary opinion-based doesn't necessarily need to be deleted since it is salvageable: It is enough the OP removes that part that makes it subjective, and asks an objective question.
I would rather use that flag for low quality closed questions that cannot be salvaged or have never been edited from the OP in a way to raise their quality. (This means that an edit that changes i have this problem in I have this problem, or merely adds a link, doesn't count as useful edit.)
For answers, I would use it for non-sense answers like the following one.
dfgdfgdfg etyerthe erhrjhsrjrstjrjrsjhtsrtjh.