The way I see it, the biggest problem regarding ranged damage reduction is figuring out which mobs that only does ranged damage, and how they're affected by your physical-/magical damage reduction. Seeing as there is no tab for mitigation against ranged attacks we could assume that ranged attacks are affected by physical-/magical damage reduction. Comparing reduced ranged damage to the other two, reduced damage from melee and reduced damage from elites, I don't think anyone would argue that you'll face more melee and elite mobs than you will non-elite ranged mobs. Though for the sake of not leaving anything out it makes somewhat sense to add it to the spreadsheet, however it may be a bit more complex.
Regarding the argument that reduced damage from ranged attacks only is beneficial in a few scenarios, is that because most projectiles considered as spells? Is it only like the skeleton archers that actually count as ranged attackers?