Because anything worth having is worth working for.

You could be the typical twat and run around just tapping the button and unleashing the billion Crissagrim blades, or you could spend a dozen hours with the octopi feeding the Muramasa, building up a sword that can really kick shit around.

I love the muramasa.

Fucking A, best sword in the game if you're willing to put the crazy time into it.

I've played this fucking game so many times to the point where I have every single dropped item, everything complete.

Possibly the best game ever.

how do you get the muramasa? 

I do love dual-wielding the Crissagrim, myself.  It sort of breaks the game, though, much like Alucard's Shield.

Played through Symphony of the Night fully on the PsP for the first time, I had only played the psx version a little bit previouslyand thus didn't know anything about Crissaegrim. I was just minding my own business in the inverted castle with the Heaven Sword when I got lucky. 
 
Crissaegrim is the silliest, most awesome weapon ever.