Well, one good example comes with Rainmeter.
In the built-in Illusto suite there is a Disk skin that has two variants, for 1 drive and for 2 drives. If you have 3 drives you're out of luck... What if you have 10 drives? To cover all the bases you'd need 26 possible variants (with almost the identical code repeated over and over), and the user would have to choose the one they wanted. Every time you wanted to tweak something, you'd have to fix 26 skins. Ugh.
It would be much better to create a single skin that can handle from 1 to 26 drives and configure itself automatically based on whatever you have.
Obviously, the Illustro suite is intended as a starter kit and example that people can play with and learn Rainmeter, so it is fine the way it is, but that is just one of many "single-skin" suggestions I could think of. It's more difficult to create skins that are dynamic or are driven by a set of configuration options, so putting something like that in Illustro would only confuse newcomers.
I know you use the Gadgets suite, so another example is the Analog Clock. If you check the settings, the clock can appear 9 different ways, with different faces, hands, fonts, etc. No need for 9 different skins. But the Digital Clock variant is needed, as the contents of the .ini and the appearance is completely different and shares almost no code with the Analog variant.
I know you are learning, so what you have is good. Now you can start learning about using variables to control the way your skin appears, so your fonts and animations are driven by a set of configurable options that change the appearance of the single skin.