It would also be open to single player abuse where you no longer seek out iron sites, but instead build what you need, and only seek out those resources from another AI when you need to use them, instantly bring them back to full strength. Same with horse units, although horses seem much more plentiful.
I think Civ 5 has it right that we can have spears for a mostly defensive army until muskets. Back in Civ 4, if I remember correctly, there used to be times when you have no copper, no iron, sometimes no horses, and you're stuck with ranged units all the way until muskets.
Strategic resources are strategic because they are have-or-have-not resources, unlike luxury resources are mostly interchangeable and you have multiple sources of happiness.
It is rather disappointing if we draw a start and we can't build our special unit. Perhaps special units could still be built without resources? They will come out as the usual non-special units, and still suffer the same penalties as today. They cannot be upgraded into their special unit versions even if you obtain the resource later on. They gain their special promotion when they are produced *with* the resource. Of course, losing your special resource won't downgrade your special unit to normal units, they just gain the same temporary penalty to combat as usual, which.