Why One Would Want Plastic and Rubber for a Camera?

This K-5 was cracked after a crush:-



Source: http://forum.xitek.com/thread-1000353-1-1-1.html (in Chinese)

The new K 30 looks like shockproof, with its plastic/rubber outer shell construction. So, it seems that sometimes plastic and rubber is better than metal, especially consider that the K-5 metal shell seems to be flimsy and not thick enough. This can be seen in the above photo for the wound. Surprisingly, this was exactly my own feeling when I first picked up the K-5 in my hands. In contrast, the Pentax MZ-S film body is a different story (except for the filmsy "data back" of it).