why not? you could have a kneeling stance for accuracy like in most fps.
How would kneeling with a bow make you more stable? Maybe with a crossbow...
Still it's not really a stance for the weapon, but rather the whole body.
The thing is that Warband provides a 100% weapon-proof invincible 'wall' at a certain degree in your looking direction, if you 100% block correct. So implemented feats like feinting, chambering or kicking as well as player tricks like dragging stabs, roflcopter, hiltslash, 360 turns etc. have the function to break through that wall.
Melee tries a whole new approach which the system of stances, timed blocks, passive blocks and stamina. For example only 1 timed block will force a stance change on the opponent so he can at max. achieve a passive block in response to your following attack. At this point the whole block-slash-block-slash rhythm we have in Warband is already broken. How that will work out or feel to play I have no idea but we have to disregard taking basic mechanisms from warband to be granted.
That would imply being locked into animation once you attack, not being able to move or rotate at all. I don't like that, it would completely ruin the fluid feel we had in the first alpha testing.
It's not a valid tactic irl though, as is feint spamming like you do in Warband.
Both of those are valid tactics. Any tactic is valid, if it can suprise or give any kind of advantage, over your opponenent.
What isn't valid, is your assumption, that just because you don't know how to pull something off, nobody can.