[Agema]

Resources sort of *are* gold.

If you boil all issues down to survivability and killability, a 10GP/20Res unit is far better than a 10GP/1Res unit. So you could view a Res-heavy unit as worth X Res-light units. In every game I've played in even in the midgame there are still large armies being thrown around. Shields and armour are still making troops more survivable from all manner of spells, arrows and so on, and decent weapons are still enabling troops to kill enemies faster.

Consequently res-intensive units save you gold because you need fewer units to make an effective army, and save more money because you don't have to replace losses to the same extent. That money can go to castles, mages, and all the other funky things money can buy.

Yes, Res becomes steadily less useful as the game goes on. But it allows you to pack more punch for an equivalent gold cost (or pack the same punch for less gold), which makes it potentially a very useful tool for a head start.