I don't use a skin or shell. I did think about something for the bottom of the MBA to insulate myself from the heat when the MBA got warm. But for what I am doing recently, the MBA stayed cool enough that I didn't think about it anymore.
I am reasonably careful with my MBA, and the thing is make out of aluminum, not wax or some sort of soft material, so I think I will be OK with the general wear and tear.
Besides, it is a shame to cover up such a beautiful and thin design with plastic or glorified vinyl (the so call carbon fiber stuff 🙂. Well, I am being a bit bias because I don't think too much of carbon fiber on a laptop, but a wood skin may be interesting. So yes, I see that I would consider a skin as a decoration, but not a thick shell.
Back to the bottom heat, does any one know of a thick skin like material that is designed for the purpose of keeping the heat off your lap? I am not talking about lap desk (I am making one for myself).