Provide sharpening previews

What problem do you see this solving?

When applying sharpening via the sharpen sliders, it is sometime hard to visualize the effect each slide will have. Lightroom provides an edge highlighting feature when holding down the option key. With each slider, you can preview what effect it will have.

When was the last time you were affected by this lack of functionality, or specific tool?

This feature would be helpful every time I add sharpening to an image. But it would be of particular value in returning sharpness to an image that has hade noise reduction applied.