You still could detect secret doors manually - it would just be harder as you'd have to find the switch. With the skill, you notice things that have been disturbed (keys under ro1cks) or that don't fit properly (inconsistant stonework for a secret door).
Also, the game is a role-player and people who don't have the patience to scan for secrets can play a character who can. The Know Creature skill is another good example. You can work out creatures for yourself and keep them all written down on paper and save a few skill points. Or you can play a character who can do that for you. I'm a scrawny b'tard in real life but I love swinging axes!
