When used solely as an attempt to elevate what would otherwise be a mediocre rendering, though, it becomes a crutch just like using an Instagram filter to elevate a mediocre photo. Really dig into your materials, lighting, and geometry, and you'll eventually find that a noisy rendering will generally look inferior to something more polished.
Exactly. Also a great point on photorealism vs realism, Louis.
The comparison of noise vs. no noise is a good example, you can see it adds a more realistic and warmer touch.
For a better effect, use slightly less (-0.25/0.5%) noise and whiten the background more so it becomes cleaner.
The best thing to do instead of noise is making materials more realistic with a slight bump and specular texture especially on wood and an added opacity texture for fabrics. Then what I do sometimes, a little Spielbergian, is adding a warm light panel on the side where light hits from the top, and a blue/purple one where the drop shadow is. See an example of my recent project here: http://designsoul.nl/comodo.shtml
I find these colored side fill lights very effective. You can also spot the 1% noise here!
However for quick and dirty concept renders that have to be done in a few hours, including the CAD work, only adding that bit of noise is a way to improve workflow.