There are no binds in this game at present. And the way the shield works is greatly simplified: click right mouse button, and you raise the shield to protect yourself. You can still be hit by missile weapons to your legs or melée from the back, but you are impervious to any and all melée attacks from the front, so long as the shield holds. To compensate for this, the shields are more easily broken, and, more importantly, you cannot attack with your shield raised. When you attack in M&B, you expose yourself. This is not true to reality, but then a shield would be completely unbalancing otherwise. And that is why a shield bind won't work in this game either, as you are stuck with one of two options:
A. Either you will end up in a pointless stalemate, as neither you nor your opponent will be able to attack (and what do you propose as an in-game counter to a shield bind, anyway?), leaving both of you vulnerable to attacks from third parties.
B. You add the ability to attack while blocking, which will completely unbalance the game unless you make shield dynamics VERY advanced, with the possibility of being hit by any weapons even when blocking with the shield. If you start adding realism here then you'll have to follow up by adding realism there to compensate, or you will create an unfair (and indeed unrealistic) advantage. And once you start adding realistic elements, you'll find you'll have to keep adding realism until you have something which is essentially unplayable. It is one thing to coordinate one's whole body in various moves of varying complexity, but quite another to translate all those possibilities to mouse and keyboard.