There are some units in Age of Sigmar, Skaven Stormvermin for example, that says that "some units cary shields into battle." The units ability to carry a shield is not restricted by any weapon combos or special abilities, and the shield gives a flat +1 bonus to save rolls when the unit is equipped with them. So what reason would you 'not' equip shields to the unit since it is just better? Is it only because of the physical models that may or may not have shields on them, or something else I missed?
Age of Sigmar is very much designed with the "beer and pretzels" mindset. The main reason is that you may have a bunch of models without shields. If your opponent doesn't care, you can always say that all the models have shields.