One advantage of the NES Advantage is that, due to its size, it's easier to use with your feet. Yeah, I know, it's ghetto to play Karate Champ against yourself, but what do I care? Karate Champ was fun! And I loves me some of it.
Seriously though, this thing became invaluable to me when I was playing, 'the cheating way,' through Megaman 3 I don't remember what you had to hold, I think it was right and A, to jump super high, but I know that my dad didn't want to do it anymore, my sister wouldn't come near the NES unless it was a game with sprites the size of my head, and mom was reading a book. I sat down on my butt, and the realization of the true beauty of the NES Advantage finally dawned on me. I went to town.
The NES Advantage also has the ability to help you more than R.O.B. at Gyromite, and functions as an excellent paperweight, an interesting piece of gaming paraphenilia, and snaps those caps off of any jug, bottle or jar, guaranteed.