On a practical level, I enjoy playing the NES with different types of controllers as each one has a different feel and personality to it. I used to practice with nunchucks regularly, and during this time I bought different types of nunchucks because each one felt, weighed, and twirled differently, changing my performance depending on which weapon I held in my hands. Some nunchucks were better for this and others were better for that, while some were just novelties. Apply the above statement to controllers and joysticks.
Aesthetically speaking, each different controller in my collection is like a different flower in my arrangement of NES controllers. Although a Famicom controller may more or less play just like a regular NES controller, the look of the controller is unique, beautiful, and even exotic due to its foreign origins and limited availability here in the States. As a collector of the NES, I seek out such rare and unique additions.
The creative process (if it can be termed such) of making a Famicom controller work for the NES will be fun!! I'm no Ben Heck or Richard Daluz, but I enjoy tinkering around and modding stuff with what little bit of electronic ability I have.
Finally, I collect Famicom games, which I play with a converter, so it just seems like an obvious step to mod THE Famicom controller to play my Famicom games with.