As others have mentioned, there are a few good reasons to disable the Badass ranks:
Single-Player: While the stat boosts seem marginal at first, if you've already played through a few times already, they start to add up. While I personally enjoy this, it can make starting over with a new character much less challenging, especially through the first half of the game.
Co-op: If you have a friend over and are starting a new game, it would only be fair to play on a level playing field together, not just for dueling, but for overall gameplay, especially if your friend is new to Borderlands: With the ranks on, you would not only have the advantage of playing experience but all of the extra stat boosts as well, which could hamper the fun for a friend playing for the first time.
That all said, I love the Badass ranks and think it was a great improvement over the first Borderlands in terms of replayability.