Plastic is usually scratchy. Glass is much better about resisting scratches. I believe you're, right, modem phone glass is pretty flexible. It's also (usually) coated with something that prevents finger oils from building up easily.
Still, the choice of glass is clearly cosmetic (less prone to scratch, nicer look, premium feel), buy technically plastic presents more advantages, it is lighter, absorbes shocks, fairly durable, much cheaper. To make for the most reliable device it would be better, but we know that's not a priority for any of the manufacturers, and propably also for many customers.
Honestly, modern phones are pretty durable and screen replacements are pretty cheap.
My complaint is that the glass backs are slippery as fuck. My iPhone was constantly sliding off of tables and shit until I bit the bullet and got a case for it.
I do not agree, owned like 3 phones all of them ending up in cracks, yes I am a clumsy person but I also work in a hangar where at one point my phone screen cracked due to carrying something.
The good thing about the glass backs is it's quite sticky on your fingers, which is nice. I'd still take plastic or something else instead, but hopefully also sticky. I'd rather not drop my phone at all, but if I do I don't want it to crack.