There is one. It's the armor check penalty, when non-proficient it applies to a lot of things that it doesn't for proficient use, including attacks (don't remember what else off the top of my head).Sigurd said:I wouldnt allow using a shield unproficiently without a proficiency penalty.
However, with the mithral buckler the armor check penalty is 0, which makes it a very popular option, the armor check penalty reduction of mithral is one of the primary reasons that people get mithral armor/shields, with armor the category drop is also a primary reason.