We can consider that someone with a higher OB has a greater 'activity' for each combat round, thus giving him th opportunity to improve offense and defense at the same time.
As for using a shield, maybe the best way to simulate this is through use of combat styles. Nonetheless, I don't think that ducking behind a boulder can be considered similar to deflecting an attack with a shield. IMO, in the first case, you just make a moving maneuver to see whether or not you gain any benefit from the cover. In the latter, you use your shield actively like any other weapon to avoid being hit and OB reduction seems the best way to do this for me.
As for cover use, I agree with Randal Thor. A huge boulder can provide full cover (impossible to hit) if you don't want to see anything either. If you want to take a look at something while hiding you have to be at least partially exposed and if you want to fire an arrow at your foe, it is even worse and then only light cover will be provided.