The Energy Sword might have a lunge and insane tracking, but it doesn't have the sheer killing potential of the Gravity Hammer. And the Halo 3 Hammer is unabashedly the king. It's grimy, it's a little wonky to use, and it creates silly ragdolls. But those are just three reasons we love it.
Without the Gravity Hammer, we wouldn't have Griffball. People wouldn't have the memories of smashing their friends into the dirt in a custom match, and all we'd be left with is a cross-map energy blade that makes zombies.