just imagine, you are working in a illustration with multiple layers and you can change just one layer to outline view mode.
this would save computer resources.
also think that you might be tracing an image on a layer above the picture perfectly visualizing the photo on layer below.
Macromedia freeHand, Illustrator and the OSS scribus(i know isn't a vector drawing software) have this option(the icon before NAME label).

the descrition of this feature in scribus wiki :

Outline mode for layer objects: This option comes in handily if you want to speed up the performance of Scribus, as all objects on a layer are only displayed in a "wireframe" mode. This is especially useful when a layer contains complex vector graphics.

a example using illustrator
https://www.youtube.com/watch?v=KRf31Y7utcI

PS: sorry for the bad English folks

Didn't know why to use such until I saw that delightful lady you just posted.
It makes sence now as those blurrrings can melt the computers.
Good idea!

Nice idea!

i create two images to illustrate what i'm telling :

Yes. Outlining by layer would be a very useful enhancement.

I have a suggestion to make it outline layers faster when working with lots of layers. To save you switching the modes on and off on every layer.
New command button: "set all but the Current layer to "outline".
You get to see the current layer in full and all other layers are in outline. So it changes the state of all the layers with one click. As you work on different layers the visibility state follows you automatically.

Activate the mode:
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Select a layer. Instantly all other layers are in outline. Work on that layer.
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Click around on all the layers to see their content.
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• Review the content of each layer with 1 click on each one. Fast!
• Optionally, other layers can be completely hidden.
• If you switch this mode off, the original visibility state of the layers is restored.
• The "ticks" in the list never change. They are just temporarily overridden.