Sahbumnim told us that his teacher didn't believe in conditioning the hands by punching a makiwara or similar toughening techniques.
"He said the calluses block your ki and keep you from striking your opponent with maximum force," he said with a bit of a smile.
In my usual contrarian fashion, I found myself wondering in what sense this might possibly be true. (Maybe it's just resistance to authority; I think it's more likely the years of literary and Biblical criticism. Following a downblock with a reverse punch isn't the only thing that can become a conditioned reflex.)
I came to the conclusion that there is a metaphorical sense in which toughening your hands might indeed restrict your ki from striking your opponent. This may or may not be what the grandmaster had in mind.
If you're working on toughening your hand in order to protect it from injury when you punch, you're thinking of your hand as a part of yourself to be guarded from injury. You're not thinking of it as a weapon.
The difference between "How can I protect this delicate part of my body from damage?" and "How can I use this versatile tool to do maximum damage to an opponent?" might well make a difference in your fighting style.