the simple question, what does this open claim add to the game.

Not multiple claim, as in 1 group being able to claim more than one mob, because that should have been in place from the beginning.

What does being able to attack a claimed mob (because it is claimed, in terms of every possible benifit it could give you) add to the game?

nothing, except maybe an easier plevel.

What does it take away? Well basically overworld NMs and NM mechanics, would have to be built with the claiming system in mind, and all signs point to them doing nothing different about overworld NMs and overworld fighting mechanics to take advantage of the system they propose.
SOOOO essentially all it will do is create problems with NMs, or cause them to make overworld NMs cease to exist.

So overall, the system does more damage than good, and is a waste of development time.