I have always made it through the day at 30%+ on my Series 2 Nike and my Series 5. My Series 0 was pretty bad battery wise.
I often can get through the day with 30% left on my Series 5 (cellular model but with cellular unused) with AoD turned on. However, I sit at a desk all day and perhaps I interact with my watch less than some people. For example, if I'm at my desk and see a notification for a text, I'll pull up Messages on my Mac to read the email than try to read it on my Watch. Plus, if I need to take off my Watch for whatever reason, I'll put it on the charger so sometimes I'll end the day with the Watch almost fully charged.
OTOH, my wife says that battery life sometimes doesn't last the day for her, also with a Series 5 (cellular model but with cellular unused) with AoD on. But then again, she's not always the most disciplined with keeping her Apple Watch charged. She'll be home watching TV after work and then complains later that her watch is out of juice for the dinner party later... because she didn't bother charging it when she was home not using her Watch, even though I put a charger right in the TV room for this reason.
Ideally we wouldn't have to pay attention to charging these devices, but it is what it is.
It's not at all needed on watches either. You have to move your wrist towards yourself to see it anyway, so just use that and you avoid wasting battery.
While I agree it is not necessary on a Watch either, I think it's a nice thing to have on a Watch. Personally I find it annoying to have to move my whole arm just to take a quick glance at the time. AoD would be nice on an iPhone too if you didn't have a Watch, but for those of us with an AoD Watch, it's less critical.
BTW, now that I can unlock my Macs automatically with my Watch, I don't really care about FaceTime unlock or Touch ID for the Mac. Too bad this Watch-based unlock feature isn't supported on the iPad.