Can someone explain this whole aluminum vs. plastic vs. glass thing to me?

I like plastic. It is one of those modern miracle materials that is strong, light, moldable, cheap, and permits a clear wireless signal. It can be sanded down like on the Lumia 920 or One X and it works quite well.

Aluminum is heavier, blocks a wireless signal, and in HTC and Apple's cases, completely seals your device so you can't get inside for storage or the battery. It doesn't really seem like a "quality" material for a phone IMO