It puts you too close to a barrier to be able to manuver quickly, forces you to pull your weapon off target in order to move, and believe it or not exposes more of you to the target.
See how close I am to this tree? If I have to move to the other side, or even move to the right for any reason, my rifle is too far forward and would have to be raised to allow it to pass, or I would have to move backward. Shooters being this close to cover also have a tendency to use the cover as a rest. This is bad juju in that if you use a tree, or other wood cover, and your hand is resting on it, you're going to tear your hands to shreds if you're not wearing gloves. It isn't the most accurate way to shoot either...
Now I am further from the tree allowing for more manuverability.