Skins are used to replace the original textures of a model with new ones. I think that what you're trying to say is that skins look useless, but they aren't. Sometimes, when you want to make a big map that you need all the original models, you may have difficulty using the skins as you can't use the old model's textures. Althrough if you are making a map like fel orcs against Humans, you can use (for example) Afro's skin of a fel keeper etc. You may realise that skins can be very usefull, since they don't take so much space on your map. An example is:
Your making a DotA map of Fel Orcs vs Humans (like I said before) and you want a fel keeper. Instead of using a model that will take like 30-60KB for the model/portrait and 40-90 for the textures for that model, you can simply import new textures for the already existing model, which saves up much space.
-Rui