I would argue that all of positives you cite can be accomplished without trolling. What distinguishes a thoughtful but contrary comment from a trolling comment

I would argue that all of positives you cite can be accomplished without trolling. What distinguishes a thoughtful but contrary comment from a trolling comment is that the troll wants to force other users into negative behavior. Trolls win when somebody blows up, or somebody's self-image is toppled, or whatever. That attitude adds zero value to a conversation. Whether any of this matters or not is immaterial. If someone thinks it matters and you want to convince them otherwise, pushing them to drop the banhammer in anger isn't the way to do it.