Hope this isn't too stupid of a question...
Is there any reason that a polarizer should not be used in place of a neutral density filter? Of course, the polarizer will have an effect on skies and reflections. But I suppose I am asking, if you're shooting a scene where a polarizer doesn't have any effect, is there any reason not to be lazy and leave it on the camera (assuming you used it previously) rather than replace it with a ND filter?