Bunkbeds are wonderful things. (Ask any college student who is trying to eak five more square feet of floor space out of a tiny dorm room...)
When I was four years old, my father decided to make a bunk-bed for my older brother and I. Even better, he let us help him--we held down lumber while he cut, we learned how to fasten boards together with nuts and bolts, etc.
My mother also had some worry about boys falling off of bunk-beds--so she insisted on having a railing that could be attached to the bed-posts.
A year or two after we were large enough to mount and dismount the railing ourselves, we decided we didn't need it. It spent the next decade or so languishing in a corner of the room, or the closet. (The only time anyone fell was when he was leaning over the side of the upper bunk, trying to smack his brother on the lower bunk with a pillow...)
I do agree--bunkbeds seem to be the kind of thing that young boys (and fathers) see as nearly harmless, and mothers see as dangerous.
There are few events from my childhood I remember quite as well as the day my bunkbeds arrived, and, if my son's joyful reaction is any guide, it'll be a day he long remembers.
I think I did fall out of the bottom bunk once or twice, but never the top bunk, which had a pretty meager guard rail (which nevertheless was sufficient). But like you, I got rid of it by the time I was seven or eight -- it's still in my parent's house, in the basement (the bunkbed is there as well, but, alas, it's set up as twin beds, perhaps because the ladder, of all things, fell apart).