I've already dropped my S8 once, and, just to reaffirm my trust in the universe, is there a rational reason for making the back glass? Note 4 had rubberized plastic on the back and while it maybe didn't exude as premium a vibe as S8, the thing stuck to your hand and was a pleasure to handle.
Is Samsung being logical or trendy?
2 reasons:
1: Wireless connectivity which includes wireless charging support and Samsung Pay (Sammy paid $250 million for LoopPay's patented technology, which puts them ahead of all other phones in terms of mobile payments). A metal back exterior casing will block the wireless connections, this is exactly why metal phones have antenna lines. Unless they did what the Pixel did and make an awkward half glass half metal back phone, they will stick with glass (I'd prefer the S8 design over Pixel). It's Gorilla Glass, so it's just as tough as the front glass.
2: Beauty. Who doesn't think the all glass design isn't beautiful. It looks like a piece of diamond jewelry, being all glass and curved. Sure, they did sorta ripped off the look of the iconic iPhone 4, but they had a reason to.
Samsung for the longest time had been made fun of for using low grade materials such as plastic which made creaky sounds when you grip the phone (my S5 was very creaky). However, it made more sense; it allowed for a removable battery (with the option for wireless charging if you purchase a special back casing), and also allowed wireless connections to transmit in and out from the back exterior. It also was much more durable because it's plastic and can't shatter. The majority wants wireless charging, Samsung Pay, and a premium/beautiful phone so that's why we have the S8 today.