Save-As writing bunch of resources, that when loaded back into the browser might even be broken by lack of external resources are also, in my oppinion, a bad way to keep evidence (e.g. CYA). If you need it to persuade a non-computer-savvy person of something stated on a website, I'd say a bunch of random files in a directory is also much less convincing than a "printed-to" pdf, or screenshots you can easily mail around.

Regarding the .webarchive, I think IE supports another type of "web archive", with a .mht ending.

Opera and IE support MHTML (MIME HTML), it seems to work alright