As a Heavy, I have in my posession what is arguably the most powerful close-range weapon in the game. One that can shred through an enemy in fractions of a second.
I am also in posession of the biggest 'guns' in the game, them being my massive fists of doom.
Which seems redundant, given the large number of boolits I can shoot with my mighty mighty gun.
So when is it a better idea to spend the time switching to my fists for a good punchfest, over holding out my minigun and preparing for a mowdown?