I have the Moshi palmguard (which protects palm area and trackpad) as well as the Moshi keyguard on my laptop to prevent any wear or discoloration. However the aluminum does seem to stand up to wrist wear quite well. One thing that concerns me (and thus buying the palm guard) is wearing a watch while using a laptop, as that can scratch the palm area to bits.
Bundle in the fact that I sell my laptop every 1-2 years, I keep it in good shape by using these covers, ups the resale value. So it depends on what you have in mind, if you want to sell in the future they can be a good investment against potential damage, however the aluminum does hold up very well to wrist wear. We just got a 3 year old MacBook Pro from a faculty member here at work and the wrist area is still perfectly normal.