For those that don't see a use case: Chrome's dev tools work well for measuring things within the context of the box model, but so much of what I measure on a regular basis isn't captured there. The best example I can think of is measurements relative to some text's baseline—the distance between the bottom baseline of some text and the border of its parent container, including adjustments for line height, padding, and related margins, for instance. This extension would also work for things that aren't even represented in the box model, like the distance between two shapes in a <canvas> element.
One feature suggestion: it'd be great to be able to click or use another keyboard shortcut to set arbitrary anchor points on the page. That way, I could measure the distance between elements that aren't necessarily on the same vertical or horizontal axis. That's one thing that OS X's screenshot utility is great for (though perhaps unintentionally).