Time zones tend to come into effect in two situations: when you need to communicate across them, and when you need to travel between them. In the former situation, I agree that having one universal time would simplify things. Also agreed that as long as you tend to stay in one area, it wouldn't be a big deal to get used to dawn at 17:00 or whatever. What I think would be tricky is if you did a lot of traveling. With time zones, you can simply change your watch/phone to local time, then work based on that. With universal time, you would need to either constantly work out what time morning/lunchtime/business hours/etc. are in this new place, or you would have to change your watch/phone to some fictional time to simulate the times you're used to. Either one seems significantly trickier.

Of course, nothing stops people from using UTC only for the situation where it's really useful: long-distance coordination. (And as mentioned in other comments, that's often done already.)