It's kinda like what Darrell Royal, in the days of unlimited football scholarships, once said about how a certain recruit that wasn't what we'd consider a 5-star recruit now, but had potential to be a good player, would do at Texas, "I can't guarantee that he will play for the University of Texas, but I can guarantee that he won't play against the University of Texas."
What Rule 5 is trying to do is to keep teams from stashing players in the minor leagues just to use as injury fill-ins and, like Royal, just to keep from having to face them. Either protect them with one of your slots, or let them try to make another team if they're drafted.