I'm honestly having a hard time understanding you here; you seem to be moving away from the point that we were discussing before, which is whether or not saying that no point is ever beyond debating means (i.e. necessarily implies) that you're owed a debate.
I don't believe that it does.
If you're saying that you see no other reason for why someone would ever make that assertion, then that strikes me as a failure of imagination. Insofar as (+) threads go, I don't think that disagreeing with the premise is "irrelevant" or "damaging" or "terrible" (presuming that's what you meant; as I said, I found your post quite hard to understand); it's that people have a different opinion that they see value in sharing, and don't believe that it should be preemptively disallowed (or sequestered off to another thread where other interested parties might not see it).
The failure of imagination is yours here, and I'll try to explain why.
So person A, Alex, comes across a (+) thread. He, as you say, has a different opinion, and he sees value in sharing it.
The whole point of the (+) thread is that it's been declared, up front, by the poster, let's call them Pam, that there isn't
value in the opinion being shared. They're saying "please don't make X argument, we've heard it, we've considered it, we don't find value in it."
The problem here isn't that Pam simply doesn't understand that what Alex had to say had value. It's that Alex simply doesn't understand that just because he thinks his opinion has value, that everyone else should think that, too. Alex needs to learn that his opinion is neither new nor wanted.
Not all opinions have value, full stop, and many opinions can be very disruptive when expressed in specific instances. The whole point of the (+) is to cut those disruptions off at the head. To ask, request, that you not talk about X or Y in this thread.
And if Alex can't accept that his opinions aren't valued, and insists on sharing them anyway... what then is he trying to get out of it, then a debate? One he very much isn't owed.