I have been taking styro/paper coffee cups on day hikes - I test stoves and other gear on long day hikes. A hot cup of tea can be bracing when it's cold out. Backpacking I still put an insulated mug on a biner on the back of the pack.

I'm more careful with my gear than other people. Generally it appears people use gear, toss it around, drop it, and get irritated when something breaks or rolls away. For people like that maybe they're better off with bomber packs and stainless steel.