In this case, it's less about discoverability, but more about long term archival. Will daemonology.net continue to exist forever? Arxiv.org might perish, but I am sure the community will make sure the data will be preserved.
I'm not too worried about that -- this paper is "mirrored" on hundreds of university websites since it's a common reference for graduate courses in computer security.
In my experience, once teachers retire or move on, or a course gets mothballed, it's only a matter of time for course websites disappear or become non-functional.
If the course website was even on the open web to begin with. If they're in some university content management system (CMS), chances are that access is limited to students and teachers of that university and the CMS gets "cleaned" regularly by removing old and "unused" content. Let alone what will happen when the CMS is replaced by another after a couple of years.
I wonder if there's an Aaron Swartz of paywalled university course material out there? Someone (or a group) downloading, datahoarding, and sharing every collection of courses they can get that you need a university login to access?
