I just have to say that I’ve had a blast with the idea to use other players as enemies to the main party. More people get to play and get a chance to be antagonists. 
     The monsters are maximized and DM battle management must be more refined, yet players get a proper challenge and I can focus more on the storyline and other plannings.
     The monster be mature enough to understand that their part is rather futile and along the lines of the show “Faster, Purple Worm! Kill! Kill!”. Both sides enjoy the fun and I get to see more personality and behavior devoted to the monsters to act more like they should. 
     The only downside is the room needed to accommodate all the players. Hahaha! If anyone else has done this, I’d love to hear your fun experiences….

I am DMing a campaign with some friends that's had only a few sessions so far. My brother wanted to be a part of it as well, but due to scheduling issues would not always be able to make it to the sessions, so we came up with a fun solution. He plays a character much higher level than the party that shows up sometimes to antagonize and bully them. I've made it very clear to him that I will strike him down if he tries to kill the party, but adding in a little bit of mischief here and there helps to spice things up and gives me some time to relax and let the players handle the challenge while I focus on getting other things ready. Thought it was similar to your idea and worth mentioning here.