Cosmetic defects are not common to refurbished Mac products. Apple doesn't guarantee that there won't be any, but I've never seen that happen and my friends and I regularly buy refurbished. My refurb Mac Pro has been awesome for years.
Officially Apple says:
As these refurbished products have been unpacked and manipulated, they might however exhibit some minor cosmetic imperfection, such as scratches, marks or discolorations.
If you are very concerned about the possibility, call and see what the exact return policy is.
It's a little different with the iPhone/iPad where Apple says they replace the outer shell and battery. They don't actually say that for the Macs.
No customization is allowed. These units are already in the shipping boxes and they will not open the box to add anything.
The main difference I know of is the D.O.A. policy. A refurb that is dead on arrival cannot be taken to a local Apple store for service, it must be returned to the source.
One you are past the D.O.A. period, it is serviced just like any other, and the remaining warranty and AppleCare eleigibility is identical.
All the refurbs I bought (2 Mac Pros and a MacBook Pro 15") were pristine. The only difference was the box was blank white.
A MacBook with an ugly scrape does not meet Apple standards for new OR refurbished. It would have to be repaired before it could be sold. If you are not happy with what you get, you are free to return it within 14 days.
Grant, greetings: You said 'No customization is allowed.'. Not true but true. When I ordered my refurbished MBP I remember that there were a couple of upgrade options offered. One was for RAM and the other i believe was for the HDD. However the prices for these upgrades was MORE than what Apple charges for new ones.
Don't hold me to this, but I think for an 8GB RAM upgrade was $400 (maybe more?). I just laughed and wondered who would pay an outrageous sum for a RAM upgrade when the whole point of a refurbished unit is to save money.
So after all it really is not true unless you want get a refurbished MBP for the price of a new one! Then again P. T. Barnum was right.
Wishing you a good New Year,