Ventilation
I'm just not understanding how those vents would work without corresponding vents and fans in the computer. As I understand it, if the bottom is on a table, it takes the left over heat the ventilation system doesn't take care of, (and it takes care of it pretty well), and acts like a heat sync. That's what all the people at Apple have told me.

Perhaps I'm mistaken.
I liked the Speck until I had to take it off. And I'll say that I also felt like I had a "bigger" computer, in that when I took it off, all of a sudden, I remembered why I had purchased this beautiful, slim, light form factor, and why would I want to bulk it up?
If it gets scratched, I figure those are distinguishing marks, like a beauty mark, or a scar. Oh the memories... (Well, unless you're careless and bang the screen in a door).
It just feels lighter and more pleasant. I'll never use a case for a laptop again. It hides the built in beauty, and just adds bulk, IMVHO. (And I don't believe the "vents" in the cases are useful for more than a placebo effect.
I have a soft case which can fit in my computer back pack when I need it. That's what I use.