If you level multiple classes at once, and have several sets of gear to keep track of, sometimes people make mistakes. It is indeed not that hard to prepare ahead of time, and people certainly should prepare ahead of time. But if I end up making a mistake, distracted during a long play session, and I have the equipment to repair my gear right there in the dungeon, what option do you think is best:
A) My ability to repair my equipment is arbitrarily disabled and my simple mistake ruined the dungeon for other players. They could have been waiting in the DF que for a half hour and all that time was wasted because one player - who they had no control over - made a brain fart.
or
B) I can use my ability to repair my equipment that I have in all the other parts of the game. I end up not wasting anyone's time and SE's gameplay mechanics have thus fostered a happier, nicer community that isn't depending on each other to never ever make mistakes that have nothing to do with in-combat gameplay.