At least for the uses that came to my mind while writing this patch, one override is usually enough. For example, there is currently no way to render a "clay render" with activated mesh lights since their emission shader is also overridden. Multiple renderlayers and/or the compositor won't help there since the lighting itself is basically broken. With this patch, you simply move the emitters to a layer and set it as No Override.
Another example that was mentioned is rendering an ArchViz preview where glossy materials like mirrors are overridden to get a clearer render.