But the design for the shell in the picture and corresponding CAD renderings show that the back of the shell is a single piece of metal and is designed around that. It doesn't look like you can simply replace these parts with glass inserts as it would weaken the structure and necessitate a major redesign of the top and bottom.
The picture also show the thick antenna bands on the sides, with a complex plastic molded structure that show the plastic part getting even larger inside the shell, embeded in black insulation material. To me there's something more to this antenna system than just plastic separators and the thickness seems to be an integral part of it.
The recent "improved" renders seem to dismiss the whole antenna concept, just because it looks better and generates page-views from those who hope that the leaks were not the final design.
I don't think there was ever a leak of iPhone parts that differed much from the final product. Those leaks seem to happen at a stage beyond prototyping. The only thing we've heard about the glass inserts come from hearsay in a single report, all the rest of the evidence (like this picture) show otherwise.
Here's another high-resolution picture that hasn't been published by the usual American blogs as it leaked during WWDC (9to5mac did publish a smaller version but mixed up with blurry pictures of mock-ups from a known faker.)
I doubt at this point that there will be any major changes from this so I wouldn't get my expectation too high that the antenna lines will be thinner :
