It is my observation that metaphors are useful primarily a tool for instruction, rather than discussion. In instruction, the metaphor can be used to introduce a broad concept, followed with further detail, which eventually supplants that introduction in the student's mind. In discussion, however, the speaker is not positioned to follow with sufficient detail before the other participants engage - the metaphor does not get supplanted, and the discussion becomes about the metaphor, instead of the following details that never came about.
I don't agree with this, but only because you are using discussion as a synonym for argument.
Instruction (or learning) does not have to be from the position of principal/agent or teacher/student. Instead, it often works well in a multi-modal approach ... assuming people that are engaged are listening.
In real life (as you later put it), I have many discussions with people in which I both instruct and learn; the internet tends to disfavor this type of discussion. Most people eschew learning in favor of argument.