Length is only good insofar as it adds to meaning. Most length on LessWrong doesn't do that. For example, I can summarize your first point as:

Length is good insofar as it adds to understanding. Humans are not logically omniscient: if they aren't very close to understanding a concept already, a mere summary of the concept isn't enough to make them understand it. They need examples and supporting evidence.

Simply putting down a disclaimer allows you to marginalize issues that others might have with your post, it makes relevant criticism superficially appear less relevant.

Is this actually a problem on this site? I don't recall seeing disclaimers being much used that way here.

Upon reflection, this last point is basically the essence of my criticism. We've just changed the game to make it more superficially rational, but that is more resource intensive and it masks the underlying mindsets that are bad instead of actually changing them.

It's much easier to change institutions than it is to change people, and it's likewise much faster to get people to adopt social norms than it is to make them change their thinking. "Change mindsets not norms" might work in a closed group, but not on a public site that has new people joining all the time.