Bad tanks will be bad. This just makes it that much easier to tell who is the bad tank.

With full fending as a paladin, tank swaps can be pretty easily ignored in m5, m7, and m8 on normal assuming your healers have boss awareness of when to queue up the bigger heals. There is 0 reason to use slaying accessories at this point. As mentioned, the slight benefit you gain would be outweighed by giving your healers even just a single gcd extra worth of dps time. Now if only they would do SOMETHING with parry. Get rid of it or make it useful. There are many suggestions and have been for years. Pick one. Don't just feed us an empty stat.