Spectator mode can be used in order to verify the winner of a match should someone decide to hold an online tournament.
I had entertained the idea of having a giant RR tournament that would last for weeks, or even months, and players would be in charge of contacting each other and playing matches throughout the week. I would have to form some sort of council that could spectate the matches to make sure one person doesn't try to lie about the results.
Plus, whenever I challenge random noobs on GFaqs to Wifi MM, I want everyone from GFaqs to witness his humiliation.
This update is not useless.
Edit: Just realized that it is completely random. My post is dumb, and so is spectator mode.