Not sure it would have a place in an eyepiece. If it replaced the aluminum housing there would be little weight savings, and the business end, the barrel, isn't appropriate for such a material. Imagine tightening the end of a setscrew into carbon fiber.

Plastic to replace the heavy barrel would make a significant difference and would be sturdy enough but might mar more easily. However, given all the whining you see here about the cost of large premium eyepieces can you imagine the howl if a $500 eyepiece were made of plastic?

Heft gives the impression of quality. Heavy automobile doors that close with a thud are one example. Considerable engineering goes into making a quality-inspiring sound. The LS460's door was engineered to sound like a heavy oak door. My binoculars are another example of the impression weight makes. The Leicas are made of various metals and have a substantial feel in the hand, while the latest Zeiss of similar optical quality but with some type of composite body feel light and "cheap" despite their price.

And the large eyepieces have big chunks of glass. The 31 Nagler has one element that's larger than the objective of a Pronto.