Tell me, would you rather see a plain old skin with only 1 color for each part, or a decorated skin with shading. Shading makes the skin more appealing in my opinion.

Plaid is mostly used because of those teen skins that are unshaded.

ES3MI don't understand the point of it. Why shade a models that at anytime can have light above or below them, has let polygons then Snes Arwings, and why most shading is just putting plaid over the existing skin.

Well think of it this way-

Let's say you have a skin, and you only have a few colors to use for this skin. It doesn't look very nice and kind of just bland. The shaders make the skin not only stand out, but make the skin look more professional. Also the light in minecraft doesn't effect the appearance of the skin, only its shadow. So shading is to make a skin that doesn't look as great as it can look better.

If you want me to put it into context, let's say you have a shirt, a plain white shirt. It's boring, it's lame, it's not expressing anything. So you dip it in colored dyes, and make the shirt colorful and cool but still it's the same shirt, only now it has been more modified. Hope this helps!

shading is used to make skins more realistic, some shadow at places that should be dark and some lighter pixels at light places.
and sometimes you can add some depth with it.