It's possibly an interesting wrinkle given that, especially for certain types of works, the vast bulk of material is likely copyrighted but not registered.
Far more then that, it's an absolute show-stopper for any potential lawsuits, as it's really unlikely any potential litigant will be able to prove that even a large majority of the images have been registered, and thus eligible for a lawsuit.
All they have to do is register their works that they are claiming were specifically infringed, and they are fine.
Well, with the registration issue. They still have to prove us of an exclusive right and overcome any argument that the other side puts up that the use involved fell under fair use, but they'll have dealt with the threshold issue of “are we allowed to sue over the works we are alleging were used".
Far more then that, it's an absolute show-stopper for any potential lawsuits, as it's really unlikely any potential litigant will be able to prove that even a large majority of the images have been registered, and thus eligible for a lawsuit.