I find that with aluminium-backed phones. And glass isn't much different
TBH - I ended up using a cheap plastic screen protector on the back of my Nexus 4 purely because it made it easier to grip than the glass!
For reasons like that I really don't see the problem with plastic, provided it's a good plastic (and not that rubberised stuff that goes horribly sticky after a while). It's all well and good making something out of what people perceive as 'premium' materials if most of them end up sticking a 99p silicone case on it anyway... /rant
I also find a lot of recent phones are
too thin in that they're awkward to pick up off the table without a case of some description.