Allowing light shields might allow a more defensive style of play whereas right now you either can continue down the one weapon path with a class like duelist, or go two weapon with a class like invisible blade."Swashbuckler" is a compound of "swash" (archaic: to swagger with a drawn sword) and "buckler" (a small-shield gripped in the fist) dating from the 16th century"
I know there have been a few improvements to this class already to try to bring it up to the level of other 'combat archetypes'. I don't see this doing much but giving it a little bit more staying power, with the option for another magic item slot. Making it light shield only gives a little more value than the currently useless light shield, and helps fulfill the idea of a historical swashbuckler.