Subtracting from the DC has a side disadvantage of minimising the impression of player accomplishments. DCs gradually get more challenging as you progress through the levels, but generally, for the things you're good at, your ability in those tasks improves more. That feeling of being awesome when you roll a 35 slight of hand check as a rogue, to steal that important ring from the BBE, for example, is one of the neat things about the game; it was a DC 30, and you got 35 because you're that awesome!
If they continue to display it this way, however, you'll go through the entire game being presented with target values between 1 and 20, and rolling between 1 and 20, and you generally will miss out on that feeling of personal skillfullness, or of improvement at all, because this methods highlights the dice more than the player's abilities - it's just your 1-20 dice against a 1-20 target number, no matter what you're doing. It will never feel like you're facing particularly tough challenges, because your roll and the target number will always look the same, from level 1 through to level 20.
The default display should always be the DC, and your roll with the modifiers added for your total.
Very well said.
