The MBP top case is anodized aluminum, and it can deteriorate. I think it's a combination of factors:
1) The sharp, 90º corner where the top meets the sides. Typically when treating or coating an edge (anodizing, painting, varnishing, etc.) you strive for an "eased" or rounded edge, because the finish of a sharp corner will tend to deteriorate first, before other areas.
2) Variations in quality/control. I had two different 2010 MBP 13"ers. The first one had quite sharp corners, the texture of the aluminum (say, on the palmrest) was noticably rougher. I exchanged it for a different one on which (while the same "sharp" design) the edges were much less sharp, and the palm rest was smoother. It was as if the underlying aluminum was tigher grained.
3) On the MBP's that I've seen photos of with divots on the edge, it was as if there was a divot in the base aluminum - I wonder if that has to do with the "grainy-ness" (on a molecular level) of the aluminum under the anodizing?
4) It seems that some people's sweat is more caustic than others to the anodizing.
I haven't seen any skins that actually wrap around that front corner, but what I did was get a Moshi Palm Guard, which goes right up TO the corner, and thus protects it to a certain extent. Something like a Speck See-Thru will add even more protection, but it adds a pound to a 13" MBP, and thickness. I bought a bestskinsever skin, and may apply it (haven't decided yet). It does cover the front edge but still nothing actually goes around the corner. I think the Moshi is a good first step and is fairly unobtrusive.
Of course some people will never have a problem even with no protective measures. They probably got a machine on the high end, quality-control wise, and have un-caustic sweat.
Sailboats have anodized aluminun masts, and they, too, will exhibit flaws in the anodizing where they are rubbed over time. Not pits usually, but then they also have no 90º corners to speak of, so the rubbing is on a rounded surface (i.e a 6" diameter cylinder shape).
