Good point. In fact, the flaked barley is routinely replaced or amended with flaked oats.
I kind of think of it like this: Base malts are the fuel for the yeast and provide the backbone of the beer. Adjuncts (rice, corn, oats, straight sugars, etc.) are used to modify the body and/or mouthfeel. And specialty malts like roasted barley or crystal malts are kind of like the spices of the malt world - used in small proportions to add desired overtones to the malt profile.