Can the Rights element be optional?

Dr. MacCall posed the following question in a comment on my previous post about the Rights element:  "One last question? In what circumstances would this element be optional?"

Obviously, in one sense the answer is "anytime someone wants it to be--this is Dublin Core, so naturally it's optional," but that doesn't really get to the heart of the question, which is really "when might it be appropriate or acceptable for this element to be absent?"

The Dublin Core Metadata Initiative has this to say:  "If the rights element is absent, no assumptions can be made about the status of these and other rights with respect to the resource." Based on this, I can think of various circumstances where one might leave it out:

1. Content that will never be made public.  However, in this circumstance someone still holds the rights. This element might also be a good place for a statement clarifying why the content must remain private.  If the content is merely embargoed, even if it's for a period of decades, that, too, is related to rights management and would be appropriately noted in Rights.
2. Content where the rights are in dispute.  It is understandable that a library would not want to take sides or state inaccurate information about this content.  However, rather than leaving it blank, it might be better to simply state "Rights under review" so that anyone wanting to use the image would not either assume it is freely available or expect to be able to license the rights easily.
3. Orphan works.  In the case of works where it can be assumed that copyright applies but the rightsholder is impossible to identify or contact, it would seem simplest to just leave off the rights information.  However, that does no favors to would-be users. Better to state that the item is an orphan work or use a phrase like "additional research required."
4. Content where the rights are unknown.  Again there would be the temptation to simply say nothing, but again it would be better to warn users that the rights situation is murky.  "Usage rights undetermined; users are advised to conduct further research" might work in this case.
5. Open access content.  There is a temptation here to think that there are no rights to worry about. However, even open access content frequently comes with restrictions related to attribution or non-commercial use that should be firmly stated.
6. Freely available or public domain content.  You could probably leave off the rights statement here, but why make your users do the copyright math or guess at the status when you could just tell them?
7. A rushed or understaffed project, or cases where content has been migrated from a different system.  In this situation, it might be acceptable to not have the Rights information listed immediately.  However, it would still be important to make addition of that element part of a planned later phase of the project.
8. The exact same rights information applies to everything in the library.  In this case, the digital library could probably get away with a blanket rights statement that is prominently displayed. However, the moment the content is harvested into another system, that blanket statement is lost.  A further problem could be a blanket statement on the front page that doesn't show on individual pages, so users coming directly to a content page from a search engine wouldn't see it.

To me, the Rights element has two important purposes.  One is to help protect the rightsholder from infringing uses of their content.  The other is to inform the user of the rights status of the content and, if possible, make it easy for them to request permission to use it.  By always clearly explaining the intellectual property rights of items in digital collections, even if that status is unknown, libraries can help make their patrons aware that not all content is freely available to use in any way they like, and guide them toward ethical habits of use.  The constant reminder also serves to highlight the issue of copyright extensions and how changes in the law can have a real effect on how everyone is allowed to use and re-use creative content.

In the end, although I can think of lots of situations in which the Rights element could be left out, but none in which it doesn't seem to me that it would be better to have it in there.  However, perhaps I am overthinking this and there is an obvious situation I'm overlooking.  What does everyone else think? Dr. MacCall, would you care to weigh in?