If I were a job designer... I would probably switch when PLD gets Cover and Shield Stance. Cover very rarely used or necessary; if you're holding aggro, you would only have an occasion to use Cover for bosses that target players regardless of enmity. Even then, it's not that great, since before lvl 50 bosses almost never one-shot DPS in those situations. Shield Stance, meanwhile, is a pivotal ability for PLD that makes dungeons and holding enmity very, very much easier.