This makes the assumption that the wizard would never have to make a check because he would always be in the same location as the fighter... sometimes, characters have to split upOh, the calamity!
My point is that the DC is going to scale depend on the party makeup anyways. If there is a fighter, obviously the DC is going to be higher. If the wizard is alone, then the DC will be lower (or irrelevant, if he has the time to use knock). What if there is a mixup and the fighter ends up at the door meant for the wizard? Are you going to stick with the auto-success DC?
The DM is going to set the DC at whatever is appropriate to challenge the party, nothing more or less.
