The point is, if you want to play content over your pixel fixtures, that's a matter of content selection, not a setting in your advanced output. Not playing content is also a form of content selection.
Similarly, content selection should happen on the composition/layer level, not in the advanced output. By using layer to slice routing to do this, you move this decision to a place where it's hidden from your view.
How awesome would it be, if instead of routing a layer to somewhere invisible, you could say to the layer: "only show the content in this layer where I have slice fixtures"? Your composition preview would immediately show the result of that action. It's a subtle difference, but to me it's a completely different approach that makes a lot more sense. Especially at four in the morning.