The common use of the "No copying" flag is corporate. The documents I've seen with copying disabled have usually been either proprietary corporate documents, marketing materials that weren't authorized for release, yet, and materials received from a third-party under NDA.
None of these sources believed the flag made it impossible to copy the document. However, the use of the flag means that a user copying the document has to be knowingly circumventing the flag. That makes it MUCH harder to argue in court that you didn't know you were breaking the rules when you copied a paragraph from a draft marketing announcement into a tech blog's rumor column.
I think supporting the flag, but allowing the user to circumvent it, is a nice balance between interests.
[NOTE: I'm less convinced than Jon that a court couldn't hold the flag to be considered to "effectively control access", but I don't think copyright is the real intended use of this flag.]