Speaking for myself, I think BST would accomplish two things as a tank.
First would be offering a new way of tanking, or offer something different enough from PLD/WAR/DRK that it'll stand out. In the past, I've suggested that BST's "tank stance" would be them riding their pet into battle, and your skills would change into attacks from your pet. Your max HP and defense/magic defense would also be combined with that of your pet. When off-tanking you'd hop off your pet, allowing you and your pet to fight separate from each other.
Second, it would offer a different aesthetic for tanking. Not everyone wants to be a guy in plate armor, which is what WAR, PLD and DRK are.
