As I understand it the resting potential is maintained due to the different permeabilities of K+ and Na+ and in fact at the resting potential the sum of the Na+ and K+ currents is zero. The pump's job I believe is to ensure that the concentrations of Na+ and K+ are stable and not to mantain the membrane potential directly. Since the current of Na+ out and K+ in are the same at equilibrium, an ion pump with a ratio of 2 Na+ out and 2 K+ in should work fine.
So why is there the 2-3 ratio? Does this help out during action potentials?