Like how having a copper tile is required to build Axemen? I would think it would be very uncommon for a civilization to be without at least SOME copper reserves. I would prefer if the copper resource itself just sped up the protection of the units that used it, instead of it being required. Would make games a lot more fun early on, instead of playing "guess what resource I'll get" all civilizations would be capable of defending themselves with a variety of units.