While sliding scales may more accurately represent reality, sharp gradations are the only way we can come up with a consistent policy. Abortion especially is a case where we need a bright line. The fact that we have two different words (abortion and infanticide) for what amounts to a difference of a couple of hours is very significant. We don't want to let absolutely everyone use their own discretion in difficult situations.

Most policy arguments are about where to draw the bright line, not about whether we should adopt a sliding scale instead, and I think that's actually a good idea. Admitting that most moral questions fall under a gray area is more likely to give your opponent ammunition to twist your moral views than it is to make your own judgment more accurate.