Cloud and squalls swords seemed to do just fine against heartless and whatnot in hollow bastion...
The game developers could have easliy not have made the keyblade the main wepon of the game, using some other item or skill to unlock doors. It just looks better, and the parents that happen to walk by the tv while their 10 year old is playing wont be as alarmed to see a boy running around bashing things with a giant key, then hoisting a big sharp broadsword and decapitating everything.
