My powerbook (I'm assuming same casing material as MBP) doesn't get scratched up at all. The case is pretty sturdy, but I do take descent care of my equipment. I had originally carried it in a RadTech sleeve, but then decided that it would be fine without the sleeve and just put into the laptop bag.
Then again, the bag I used was a Brenthaven, and I have been able to accidently drop my bag several times without any damage to my PB. If you want to carry it under your arm without a laptop bag, sleeves are nice - not only to protect against scratches, but to give you some grip. Last thing you'll want is to accidently have the MBP slip out from your fingers.