Because of it variable nature, the heart is unpredictable. If full of blood, it may virtually explode when hit by a bullet, while if empty of blood, the only damage showing can be a hole the diameter of the bullet as it passes through the muscle tissue. If you hit the heart, all parties have done their job.
I would agree that a bullet that fails to penetrate to the vitals has failed, my sad experience with the Sierra BTHP 85 grain .243" and 90 grain .257" bullets would confirm that. In some circumstances, when shooting a trophy grade non-dangerous game animal, any shot is better than no shot, even if the shot is unlikely to kill, only wound and slow the animal so that the hunter can make a follow up killing shot. I'll leave the question of ethics for others to answer, but few hunters are going to pass on a great trophy, even if the shot opportunity presented is marginal.
