Don't forget that the rain doesn't "splash" its way up that tube, the engine is actively sucking it up the tube. Have a similar issue with the stock airbox of our Liberty (which is kinda similar to the stock MJ box). We swapped it out for the v6 version (has an extra bit of tube that pulls air from higher up) and now we don't go through airfilters practically every oil change. :banana:
But, I guess if it never rains, there's not much rain to get sucked up. :D Still dirt/dust though.