Yes because if you want to block everything via hitting your keyboard, it means your already overloaded left hand will need to be able to press and hold for the correct amount of time any of the four keys you will choose for blocking at virtually any time.
Well, a Fang or something would make that easier than a standard keyboard.
Also my point is, a game shouldn't be hard because the controls are bad. Failing should result from an error in your reasoning and reaction, not in the way you communicate it. For this reason, there's no point limiting people in the way they modify the controls to suit them.
The warband controls aren't bad. Actually, for the amount of flexibility and dynamic nature of the combat in warband its amazingly well designed imho that you only really need LMB, RMB, WASD, and MOUSE MOVEMENTS. I like that blocking manually means that even though I see the attack coming from a certain direction and go to make that block, I could mess it up because I made the wrong mouse movement and blocked down instead of left or something. It's frustrating, sure, but it's something that helps separate players and add to the overall skill curve.