Whenever I'm using Outline mode, it's always to trace an image. I prefer to use Outline mode because the "visible stroke" is as fine as it can be, and there are no styles applied (i.e. no fills, strokes, gradients etc). In my opinion (and my usecase of course), having the "Outline" view mode seems incomplete without being able to set an image layer as a background.
I don't think every image layer should be visible, but there should be a way to change an image layer's view setting to "Traceable" (or something similar) so it is visible even when in Outline mode. It makes sense for this "traceable" layer to be slightly opaque just so the outline can be clearly seen on top. The opacity could be set on the layer, or it could be set as a global setting (whatever's easier).