I am a huge proponent for screen protectors, especially the tempted glass variety. Are they needed? Yes... as least in my opinion. Corning has done a good job with Gorilla Glass.. their production techniques have helped improve the toughness of a material that is often used to describe something that is, well, not tough.
BUT... the world is filled with crap hellbent on scratching the heck out of these displays. Some of the most common substances we come across in our daily lives will scratch even the best treated glass... all it takes is a single speck of sand in your pocket to wreck a screen. While the metal used in keys is softer, they can still scratch a screen. I've had the metal end of a USB plug scrape up a screen before. And those light scratches might seem benign, but glass's strength comes from its structure. Even a small scratch weakens the overall strength of the glass.
Screen protectors are cheap... you can get a good quality tempered glass protector for under $10... granted, I don't know how well they will fit the S8's screen. TG SPs love screens that are flat, edge to edge, something that even the 'flat' regular S7 was not (it had a very slight bulge). But let's assume there will be good ones that fit well without any of the halo bubble nastiness that poorly fitting ones have. A good tempered glass protector feels great... it's glass... you most likely wouldn't be able any difference in feel between it and the actual screen. Not only do they protect from scratches, they offer up some level of impact protection as well. I dropped a TG'd phone on my porch once, it landed pretty much face down. I picked it up and there was a pretty good crack in the protector and a chip from its edge. I peeled it off, fearing the worst, and found a completely pristine main screen.
If you do a number on the S8's screen, how much is that going to run you? Even if you have insurance through your carrier, the deductible can run upwards of $175... that might seem high, but the screen itself goes for even more than that, not including labor cost. The S7's screens were hellishly expensive, I would guess the S8's are no less pricey. Scratch up the screen protector? You are out $10.