Actually DOF scale is more a convention than some hard fact, and Nikon is marking this very optimistically so you should probably read it a bit less of what it says, for example marking 16 will give you sharp images at f/22 at least in my opinion.

Yes, that's exactly what I meant.

Thanks!

You're right, Blue. I should have said 'one of the reasons'...

Incidentally, the fact that different color light gets bent differently is why you have so many elements even in prime lenses

Actually a single lens doublet (two different types of glass cemented together) can fix chromatic aberration on its own. The reason even primes have so many elements is mostly because of other aberrations. You can't really make lenses that fix more than one type of aberration, so you need a lens for distortion, for spherical aberration, for astigmatism, etc.

Good explanation for the distance/DoF scale though :).

So, if I'm reading you right, you're talking about the depth-of-field scales printed on lenses, like the ones on this image:

If that's not what you're referring to, ignore the rest of what I say. The center line just below the distance scale shows where the lens is focused (about 1.6 meters in this case). To the sides of the focus line are marked the ranges that will be reasonably in focus when you set various apertures. As the lens is set now at f/11, anything from about 1 meter to about 2 meters (~3-6 feet) will be reasonably in focus. To the left of the lens, the dot shows where to focus for infrared photography. Glass bends low energy light (like infrareds and reds) less than it bends high energy light (like blues and violets). Incidentally, the fact that different color light gets bent differently is why you have so many elements even in prime lenses--having different focal planes for different colors (chromatic aberration) isn't really an effect photographers go for, for obvious reasons. (Although some do buy single-element lensbabies.)
The marks to the left of that center line are also called hyperfocal marks. If you've heard people talk about focusing at the hyperfocal distance (and you will have if you've heard a landscape photographer talk much), it's when you focus at a point a bit closer than infinity to get the biggest range of the frame in focus. If you set the focus at infinity, you waste some of it because the range of in-focus distances extends beyond infinity (whatever that means), but if you line up infinity with the hyperfocal mark for the aperture that you've set, you get the maximum range of distances in the frame in focus.

As the title says, I grew up with lenses that are naked.

How should I use distance scales to the fullest extent?

I know that it's a general way to guestimate what will be in focus or not. Is that it?

Plus, it's useful to focus in infrared photography, right?