Infusion cores exist for the same reason glimmer exists in infusion. To make it less accessible. There's no argument for infusion to have any cost at all other than pad out play time. As for the "grind" bungie introduced the gunsmith bounties. A "compromise" of sorts. Frankly I'd prefer they make cores easier to get than just removing them from infusion.. This would lighten the burden of infusion costs, allow players to focus more on an easier path to masterworking, and satiate bungies need for cores in infusion. It'd be a win win.