I do, do that sometimes. Like not only that, but in other ways. Like Roy/Ike vs Marth vs Link vs Young/Toon Link, the Swordsmen battle. Then the little guys, Ice Climbers vs Mr Game and Watch vs Ness vs Pikachu/Kirby/Jigglypuff, etc. Quite amusing. And the famous guys, Mario vs Sonic vs Pikachu and I'll be whoever I am..which is Peach since I don't really pick anyone else these days lolwell I usually play the same characters in franchise based fights eg. Mario vs. Lugi vs. Peach vs. Bowser (sometimes adding Donkey Kong, Yoshi or Wario) or Pikachu vs. Red vs. Lucario vs. Jigglypuff.
I only ever play the CPU. If I have a friend over or I'm at a friend's place, we'll play against each other, but 90% of the time I'm playing the CPU and I never get bored of it haha.
Admittedly, I spend most of my time in the 100-Man and 15-Minute Brawl sections because I like fending off a whole stack of enemies at once and seeing how many I can beat. I make up back stories for the characters in my head and I see the entire cast of the game as sort of like The Avengers, tasked with protecting the Earth :P
...man I'm sad.
I never get bored of the CPU either! It's quite fun. What does everyone normally like playing? Stock/Time/Bonuses/Coins?