This question came up during the game in part 3 of DD from a paladin character.
After reading all the sections on shields (conveniently split over several chapters), it seems that the only reason shield proficiency increases is so that heavy armor wearer's AC also increases when proficiency increases.
This feels lazy or awkward. Like why even include it at all? This could also further limit light/medium armor concepts with shields.
As I minor note, I do realize it helps out with shield slams but I think that is fairly minor as well.
It's my understanding that a shield bash requires martial weapon proficiency, as that's how it shows up in the weapons table.