Possible why to plastic body on 60D
As an electrical engineer with a systems-design and EMI background, I have to agree that there's no EMI shielding advantage with plastics. To get any sort of shielding you need to somehow coat the plastic with a metallic skin and even then, you don't get the same effect as a cast metal body. If you mix metal into the plastic to get EMI shielding, I've found that you compromise the strength of the material by mixing two incompatible elements.
That said, I think there are several good reasons for using an engineering plastic instead of metal in a dSLR body design:
1. Cost. No finishing steps (or fewer than for cast magnesium)
2. Part complexity. A plastic part can permit more complex shapes. witness the clamshell design of the 60D, eliminating the need for a separate bottom plate.
3. Insulation. Yes, this is the opposite of EMI shielding. With a magnesium body, there's the real possibility of short circuits between circuit boards, connectors, cables, and mounting points on the metal body. With plastic, there are none.
4. Finish ruggedness. Scratch a painted/anodized metal body and you get a shiny scratch. Scratch a plastic with molded-in color and you get a much less visible mar.
Many have complained in this forum of the cheap, plastic-y feel of the 60D. I've got a 20D and a 60D. Both feel pretty good to me. Granted, this is a highly subjective factor. No doubt there people who have picked up a 60D and reacted negatively to the feel. There are also some, I am convinced, who have not touched a 60D but complain about the feel anyway. Certainly, I have picked up cameras that felt too plastic-y to me, so I'm not saying it's impossible. As the pointed man in "The Arrow" said, "You see what you want to see and you hear what you want to hear." I might add: "You feel what you want to feel."
Shooting with Canons for 35+ years