Wulf Ratbane
Adventurer
It's only automatic if 1) you prepared it 2) you have uses left 3) it hasn't been countermeasured.
Knock gets ONE door open. One spell slot that could be filled doing something the rogue CAN'T do repetitively.
Come on. Not to belabor the point, but: Scrolls and Wands.
As soon as the wizard hits 3rd level, he's making scrolls of Knock. That's just smart play.