without wax the pores in your bases (which is high density polyethelyne I BELIEVE) will begin to shrink and dry. at a certain point the base will be unable to absorb any wax and will be stuck in a permenant state of WTF is wrong with my board.
Wax repels water, it also creates friction between the snow and your board, creating a thin layer of water which you glide upon. Bases do not repel water as well. Waxing also helps prevent serious damage from rocks and other debris by creating a layer that is removed before the base and builds up on a rock instead of it just ripping apart your base
They are 1001 reasons to wax your board, and I cant think of one why you shouldnt. Neither laziness or ignorance are viable excuses in my book
Monkey is right, I dont know that rain x is a wax, but as a liquid at room temperature it wont last long because it wont cache on the board and will not resist constant friction. Wax is a solid at room temperature and requires substantial heat to liquify and at colder temperatures becomes even more dense. Providing a longer lasting and better protection for your board. Rain x was designed for decks, not for boards.