It isn't a mainstream use, I'll grant you, but I had the 2nd Gen Sony Smartwatch (the new wear-based, despite being called No.3, is actually the 4th Gen) and found it useful whilst out training. I do ultra-distance races so, when training, I'm out for several hours at a time. My phone sits in a backpack well out of the way. My smartwatch allows me to see how far I've gone, what direction the next checkpoint is, keep an eye on whether or not anyone (important) is trying to get in contact with me, dismiss/delete the crap etc. With just a phone, all of this required me to keep the phone in some kind of belt pouch (and belt pouch sizes are not keeping up with smartphone sizes so that was becoming tricky in itself) and keep pulling it out, unlocking it and checking it. Okay, not a big deal but I'd occasionally drop it or trip over and sweaty hands/rain tend to make the screen unusable plus, until I got a Z1 Compact, none of my phones have been waterproof so I was always afraid of borking them. The watch certainly wasn't indispensable but it made a few tasks a lot easier and was well worth the £75 it cost.
However, after the first week or two's novelty wore off, it ONLY got used for training. It certainly wasn't a daily watch replacement. I can see the new ones being of use for people who have opted for a Phablet and carry it with them in a bag, though.