This is not entirely true. Not in all situations that is. There are some lenses, that in some conditions, work actually pretty darn fine on FX! And the 18-55 is such a lens! I was in the market for a wide-angle lens for my full-frame D700. Accidentally, I came across a faulty 18-55. Its optics were fine, aperture fine, and the only issue was an erratic autofocus. Manual focus worked fine. I grabbed it for only 13 euros (~$17).
As expected, the image circle is too small to cover the widest setting. But guess what? It's absolutely fine from 24mm and on, all the way to 55mm!
|Here's a 24mm example. Doesn't look bad at all, does it?|
|A close-up example at 55mm. (The dark corners are due to lighting, it's not vignetting)|
Of course, let's face it. This is a DX lens, and a cheap one, too. I'm not suggesting it can be used for serious work with a full-frame camera (probably not even with a DX one either; too slow max aperture). But it's usable if you can't afford anything else for the time being, or you don't mind the optical shortcomings.