I think that resources is key to keeping players interested in the game. Right now, once you've landed and done the science there's very little left to do. Getting there is enjoyable but after you arrive there's nothing to keep you entertained. Resources give the player a long term goal, the ability to have self sustaining missions without having to spend hours carting fuel across the Solar system and open up a lot more possibilities. They'd also give sandbox players something to look forward to and enjoy because right now, if you don't play career mode, there's very little to look forwards to.