The technical answer is yes. Primers, though some are, are not formulated to stand alone. Ceilings aren't generally under any environmental stresses, and under that logic, some do leave ceilings only primed. We had that discussion here once, and there were some proponents. There is the possiblity, even without environ stress, that it could go south in some way, such as looking dingy, fading, chalking, etc. It's still subject to UV rays. It may even cause you trouble on the next repaint. Cutting corners always comes with a price, many of which can't be determined up front. Are you willing to take the chance? The answer to that question will determine what you do.