Because abusing something that logic tells you shouldn't even physics is really skilled.
Err? Now this is an example of a non sequitur. How realistic something is has nothing to do with how it affects the skill level of the game.
I disagree. Was it something that was intended? Did the Warband devs deliberately make it that way, or is it just something flawed in the combat mechanisms that veteran players got used to?
It doesn't matter whether or not it was intentional.
Bunnyhopping in Quake 1, and strafejumping in Quake 2 & 3 weren't intentional, but they added an enormous amount of depth to the games, and they were better off for it. Those games would not have been
nearly as skill based as they were without the advanced movement systems they (unintentionally) ended up with.
Skiing in Tribes is another example. Skiing added both an enormous amount of individual skill to the game, and it increased the tactical depth as well.