Why does the joker in the dark knight use a knife?

the major one that is used in the film appears to be a knock off of a Dalton Cupid. I just bought mine tonight :P

http://sharpimport.com/Store.cfm?event=itemdetail&itemid=947787&returnto=http%3A%2F%2Fsharpimport.com%2FStore.cfm%3Fevent%3Dshowcatalog%26catid%3D68185