Shirley - Those commonly used substitutes, when there is no possibility they have another meaning, should be easy to eradicate with Editbot, and I bet they are mostly from older Gedcoms. New ones shouldn't appear. Both the create a new person and edit profile pages prompt the use of Unknown even automatically setting the current last name to Unknown if left empty. I don't know if it would do the same to LNAB when creating a profile or if it would just not allow the creation.
The fact that the WT interface is in English suggests everyone making contributions know at least rudimentary English. Unknown would be just like the "post comment" button on the profile pages, another part of the interface, used for clarity and consistency, making it harder to have the software find them all if necessary.
It is similar to the problem we have with Finnish profiles. Some genealogists insist on guessing what the people would have called themselves in Finnish instead of using the original Swedish names documented by the clergy in church books and such. Together with inconsistent use of farm names (or even hereditary last names), even when documented in birth records, makes it a nightmare to search for profiles and duplicates are bound to happen.