I prefer Swords because it's not host orientated, but someone has already listed the advantages of a hammer. It speaks for itself, but it is just a big stick.
I think the Hammer was one of those weapons that was made with Campaign in mind rather than Multiplayer, like the Fuel Rod Cannon, Sentential Beam and Fire Bombs. Brutes with no Grav Hammers is like Elites with no Swords, it's just wrong. I'm actually surprised that no one mentioned that when someone has no shields, it can kill somewhat at a distance, and you can accidentally betray teammates with it if they're too close. The Hammer could also be useful for Machinimaters (?).
A good situation is when a few enemies are in a bubble shield. You rush in kill someone, take down the shield and all personal shields and your team can br them all down.