What does it matter what his motives are, ulterior (trolling) as they may be, as long as he raises salient points and/or provides at least thought-provoking insights with an acceptable ratio?

Exactly. I often lament that the word 'troll' contains motive as part of the meaning. I often try to avoid the word and convey "Account to which Do Not Feed needs to be applied" without making any assertion about motive. Those are hard to prove.

As far as I'm concerned if it smells like a troll, has amazing regenerative powers, creates a second self when attacked and loses a limb and goes around damaging things I care about then it can be treated like a troll. I care very little whether it is trying to rampage around and destroy things---I just want to stop it.