Painting a fiberglass door?

i just purchased a 6 panel front fiberglass door for my house from home depot. the guy, when i purchased it, said that you dont need to paint a fiberglass door. however, when the guy who installed it came to my house, he said you need to paint it or it will rot away. the door is apparently primed, according to the guy who installed it. so now i have two different pieces of advice concerning the door. i bought paint for the door just in case, but i dont want to paint it if i can help it. pearls of wisdom?