For the purposes of Weapon Focus, would it be more appropriate to have it apply to a specific type of shield (i.e. Heavy Shield or Light Shield) as it is for different types of bows or a category of attack (i.e. Shield Bash) like ray and grapple?
I have a character that's two-weapon fighting with a heavy shield and a light off-hand shield, both with bashing. In this case, it's valid and beneficial to have weapon focus: shield bash, no? He will, eventually with Shield Master feat, use two heavy bashing shields.