MBP fans blow the air out the back hinge and through the keyboard, so by having fans on the bottom it really doesn't do much of anything. ;-)
I use a laptop desk for when I am using my MBA on the couch or bed though. It's mainly to make sure the blankets and clothing doesnt block any of the back vent. I also use it because it has built in speakers that are pretty decent! Logitech is who makes the lapdesk with speakers, fyi. 🙂
- Joe