I think the basic point is valid: rec., soc., even alt., are kinda more interesting historically than the more "proper" comp., sci., etc, hierarchies.
That said, I most often find myself searching (often in vain) for old posts in the comp hierarchy.
To a first approximation no one cares about bugs in some ancient SunOS version lost to history on Usenet. They care about the societal discussions that took place at the time. But a lot of people thought the former was documentation and the latter ephemera.