At work I replaced the top case of a plastic MacBook. Newer MacBook models (as in 2010 onward) are one piece, so there’s no separate top case. I learned this later from opening up the machines. Prior to that, when I would call Apple for repairs I always assumed that all that was required was replacing the touch pad, as opposed to the entire top piece.
I am generally a proponent of Apple design (and own a 2007 iMac, a First Gen iPad, and a 8GB iPod touch) but MacBook engineering, as opposed to its design, is a rare misstep. The problem is that its hand rests tend to crack when used in a ‘rigorous’ fashion, which is the only way that young people seem capable of.
It doesn’t effect usage any, but has the potential to open up the innards of the computer to crumbs or anything else small enough to enter.
It also ruins the aesthetics of the device (then again, cracks running along any device has that effect).
If I recall, Apple charges at least $400 dollars (for laptops with expired Applecare plans), so we saved significantly by doing it ourselves.
Once you get over the initial fear of taking something apart (something with lots of little screws, by the way) and proceed in a slow and methodical fashion, it’s actually not only fun, but oddly soothing.