The reason to use trapdoors as stairs is so that they can be retractable, this can be used as a design or a trap. For my example I used it as a two room house with a single hallway.
When you step onto the pressure plates the first two steps open, and you can walk through the hallway.
To get onto the stairs, simply jump over the pressure plates. If you don't want to jump every time you use the stairs you can attach the Redstone wire to a switch instead of a pressure plate, but this makes it take longer to go under the stairs.
QOTD: What sounds better to you, pressure plates or a switch?