And you are claiming that other players never allow bad plays against them or are we going to live in reality?
Safety is THE most important position in McDermott's D due to how they use them to disguise coverages and it's very hard to learn in a short time.
Which is why almost nobody has come in as a free agent or draft pick at safety and started right away. Hyde and Poyer both said it took them 2 years to feel comfortable there.
So if they are forced to use a safety that isn't familiar with how they do things and can't assimilate quickly, it means they have to greatly simplify their defense, which means their overall effectiveness goes down significantly.
So if I had to choose between a guy who can mentor the safeties, especially Bishop, on the team to help them become better, faster or some random bum who is going to get in a game and suck anyway and get maybe 10 snaps, I'm taking the mentor...he will be useful 24/7, the other guy won't be useful much at all, even when on the field.