When upgrading shields, only their attack and stability values appear to increase. As I understand the need for the latter, why does attack increase and not defense attributes? Who uses shields for attack?! This has prevented me from upgrading my fave shields, when I would really like them retain them and make them more powerful.
Since stamina management is an important part of Dark Souls' combat system, it's well worth upgrading your favourite shield just to increase this one attribute.
As for attacking with your shield... I don't really do this myself either, but a number of the enemies do.