Geoff, it's not that we're attacking you or thinking you're whinging; it's simply that, although there is no EXPLICIT Wikipedia plagiarism policy, there are Wikipedia policies that effectively prohibit it; these being copyright policy and citing sources.
If plagiarism takes the form of copyright infringement, then it has to be deleted. There is no room for argument about this. We cannot place work under the GFDL without the permission of its author, and that's that.
If plagiarism does NOT qualify as copyright infringement, then it falls foul of WP:CITE. Citing one's sources is not an option on Wikipedia; it is policy.
All that said, it would not hurt to have a Wikipedia:Plagiarism policy that puts all that in one place.
-Matt