There's a difference between premium and durable. Glass costs more to produce, generally looks better than most of the plastics used today for phones, and has more weight to it making it feel as if the construction had more thought put into it. But, as I said, I would prefer a plastic back phone, as there's less chance of damage from a drop or other impact. Glass doesn't bend or flex, which is why it's more likely to be damaged, because it's too brittle.
It's not much different in the glass used in displays. If you make it hard enough that it won't scratch easily, it'll be more likely to be damaged in a drop. Inversely, if you make it better at handling drops, it will scratch easier. IIRC, Gorilla Glass had that issue awhile back. One year, instead of the newer version being harder, they tried making it a bit softer to handle impacts, but then it scratched more easily, so the next year they made it harder.
I also prefer plastic to metal. While metal definitely has a more premium feel, no matter what OEMs say, I think it impedes wireless transmission. I live in a house with steel siding, and it drops the signal around 20dBm. Even with windows in the phone's casing, it's proximity to the antennas can affect the signal strength. And as we saw with the iPhone's Antenna gate, a person's skin can act as a conductor between the casing and the antennas when they're set externally in the phone's body.
I wish phone OEMs would use polycarbonate like the older Nokia Lumia phones. They had a nice strong casing, and looked good to boot. They were a bit slippery, but they were durable. The thin soft touch plastic that most oems use today doesn't feel much more durable than my old Droid Incredible or Galaxy S3. But, at least it's more durable in terms of drops or impacts than glass.