Holmes has this to say about dungeon doors:
Doors are usually closed and often stuck or locked. They have to have the locks picked or be smashed open. A roll of 1 or 2 indicates that a door has been forced open. Of course, if the party has to hit the door several times before getting a roll of 1 or 2, there is no possibility of surprising the occupants of the room.In other words, Thieves are door openers. They give a party an extra roll to open a door to have a chance to surprise whatever is on the other side. In addition, as they advance, they get better at hearing noise behind the door (and they begin the game on-par with demi-humans); and they get an extra saving throw when trying to remove traps (the remove trap skill). All the other stuff is fluff until the Thief gets to high enough level to take advantage of it.
The upshot of all this is that in a world where dungeon doors don't just open, Thieves become a necessity. If you don't want to face a 60+% chance of never getting a surprise round when opening a door, bring a Thief.