It was a majority of staircases, not all. And given how little room there was to maneuver regardless of which side your sword was on, I doubt it much mattered, since such stairs were generally walled on both sides. Whatever edge there was from the direction of the spiral, it would be dwarfed by the advantage from above of hacking away at your foe's head and arms while he tried to do some damage to your ankles or maybe knee. Of course, if you're having to defend an interior stair, all the important defenses have already been breached and the battle is lost, so why would you nag the architect about such a detail? This is an oft-cited "fact" for which I have yet to see any actual evidence. Until I see some actual evidence, I'll hold with the historians who call this a myth.