I can appreciate where the two of you are coming from, from a higher level philosophical standpoint. From a practical standpoint? No, really, not all opinions have value, and if a person is starting a conversation in a thread on this forum, they get to set the parameters of the conversation, and that is a very, very, good thing. It keeps the conversation on track, and keeps it from getting derailed by people who will specifically and in bad faith insist their opinions be debated. Sealioning is a significant problem in any online forum (and I mean forum as in any space for conversation, not specifically just message boards) and there is no better practical solution that allowing for and enforcing thread parameters.
Using my example from the top of the thread, if I'm looking for advice on how to make adjustments to Curse of Strahd that I don't like and know my players won't like because they don't jive super well with our values, the absolute last thing I want is for that thread to be bogged down by countless posts from people telling me not to change things and that me and my players are too sensitive and to find something else to run and I'm sorry but none of those opinions were at all helpful or valuable and they ended up derailing and getting the thread locked before I was able to get any actual helpful advice. Restarting the thread with a (+) made the situation much better and in the end I was able to get a lot of incredibly great advice because I was able to set the parameters of the conversation.
(+) threads are incredibly positive and helpful and I have plenty of first-hand experience as to why.
I feel the need to reiterate what
@Lanefan mentioned previously, which is that the practical value of an opinion is not the sole purview of the person who starts the thread.
Everyone gets to judge every idea on its own merits, and come to their individual conclusions as to how much value (practical or otherwise) those opinions have. For one person to mandate that dissenting opinions are not welcome robs those other posters/lurkers of a chance to be exposed to viewpoints that they might find valuable, even if they don't agree with them.
Likewise, the idea that the person who starts the thread gets to set the parameters for the discussion is a view that certainly has some merits to it, but which also has some drawbacks. Allowing a conversation to evolve and follow its own path can easily lead to new insights, ideas, and conclusions which are outside of the initial scope. While there's certainly room to say that such tangents should be moved to their own thread, those tangents have to be introduced in the first place, which is less likely to happen if the conversation isn't allowed to grow past its opening premise.
Insofar as bad actors go, that's an occupational hazard where discourse is concerned. Leaving aside that the label of "sealioning" can itself be thrown about disingenuously (and I say that as someone who's been wrongfully, and I believe maliciously, accused of such), the nature of a (+) thread has little to do with keeping bad actors at bay, because the guidelines it imposes have little to do with interpersonal conduct; it's more about keeping the topic within constraints than about how you act. The expectation of respect is universal in that regard (and likewise proves universally disappointing when it doesn't happen), hence why standard (i.e. non-plus) threads on EN World are no more exempt from the "don't be a jerk" rule than (+) threads are; it's just that the latter are also moderated for disagreement with the premise.
Now, there's certainly something to be said for only wanting posts that reply to the premise raised in the OP; if the initial poster isn't interested in digressions, then I can see them having no use for such things. But even leaving aside that other people might, I don't want to assign motives to people who disagree with a particular thread's premise; just because they think otherwise doesn't mean that they think they're entitled to a debate.
