Well, you don't *have* to wash it out. Some people who use it as a treatment wash it out. I do it both ways. People don't wash it out because it's harmful, but think of it as a deep conditioner AND a leave-in. If you use little enough for it not to show, you won't need to wash it out. If you use too much, it will make your hair appear greasy, so that's why you would want to wash it out, NOT because it's harmful. And no, it's not bad to apply oils to the scalp.