I agree, for longer life you want moisture to evaporate from inside the pole. You will get moisture from the top hole, the space between any telescoping sections, condensation and the ground. You need a way to efficiently expel the water though the top and bottom. Allowing it to quickly evaporate though the top of the pole and drilling a drain hole near ground level will do it. A cap will just hold moisture in and accelerate corrosion. Even though a cap may look nice, it is not a good idea.