I think it's necessary. I miss materials when I create levels. I like doing drawings with materials to decorate levels, and I need more colours. It will make levels even funnier. I'll give you some ideas:
Grass: Decreases speed, Increases grip. Round pencil.
Rock: Same as earth. Square pencil.
Magma: A different way to horribly die (And a better skin than electricity). Round pencil.
Snow: (Should stay out of winter event) Same as mud but sliding like ice. Round Pencil.
Water: (I think it would be really difficult to do) Liquid movement. Decreases speed and removes gravity. You can dive into it. Needs a recipe made of another material and acquires properties depending on it (Kills if it stays on electricity, turns into ice after few seconds if recipe ir made of ice, turns into gas if it's magma). Round pencil.
Wood: Increases grip. Square pencil.
Sand: Increases grip, decreases speed, can be removable like mud. Round pencil.
Rubber: Increases grip and speed. Bounces a little if you hit it hard.
Another idea: We should be able to apply another layer over the side skin of materials, like Path, asphalt, rails, rack...
If you consider it a good idea, I could Illustrate the skins (Just need a UV Map).