It's just fun to not be totally useless if you run out of ammo.
Say you and a friend sprayed all your ammo away and you're both stuck being some rock with a few enemies headed your way. You can't get away to find an ammo pickup so you decide to wait until they get close enough and then beat the hell out of them.
Or you just charge in swinging and hope for the best
Plus it's always nice to get a melee kill against someone who isn't paying attention.
AAAaaaaan of course it would be fun to get your whole clan together and tell them to use only melee weapons
Imagine a bunch of crazy enforcers with big sledge hammers charging at you.