I traveled with my 15 inch retina literally halfway around the world and often carried it in a backpack with tons of other stuff. It's the 2012 model and 3 years later it looks pretty much brand new to the naked eye. If you look super close maybe you see a few minor imperfections but 99% new condition.
Aluminum is just so much better than plastic for holding up over time. Plastic often can't be cleaned all the way but aluminum can.