It's a watch, I think the vast majority of people wear it without a case. I have since 2015. My 2016 S2 is still in perfect condition and is being worn regularly by my girlfriend who treats her technology awfully (she leaves it on the bathroom floor while showering etc. ) - it's fine.
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The key difference is iPhones are exceedingly slippery. Second, we are all picking it up and putting it down all day long. With a watch, you pick it up once or twice a day, so the opportunity to drop it is vastly lower. The risk factor instead is knocking into things, which is less likely to cause damage than dropping (let's face it - most people don't walk around with broken or bruised wrists all the time!). Finally, the watch is also supposed to be a clothings accessory - part of one's style. Cases mostly lack style. I can't put my finger on why, but I can't help feeling my
G-Shock 5610 (a classic) looks better than an Apple Watch with a crude case on it. Yes if you're doing a tough mudder and want to track your heart rate, wear a case, but for most people, most of the time, a case is not needed.
Also noticed that a lot of the photos of watches in cases on here also show a watch face with a battery complication, which makes me think each to their own, but it's probably a bit of an OCD thing (I used to be OCD about my battery too until I got the series 4 so I know the feeling!)