Except, instead of converting your time to theirs and thinking "oh, that's too late for them". You have to come up with some other conversion. Hmm, it's 9pm for me on the west coast, so 9pm is 9pm late or early for NYC? What is a normal bed time in NYC?

We would basically be replacing standard timezone conversion with some kind of non numeric system of identifying the time of day in a place by comparing it to normal activities. We would be getting rid of timezones and create something like "activity" zones. It would be a nightmare.

If sunlight had no bearing on human activity, and people didn't tend to follow similar patterns, then utc would be fine.

It appears someone beat me to it, take a look above. It's been a topic of discussion here before, and there are some very good reasons why this is not the case. Like not knowing what time someone might be awake in a different location, or not knowing what business hours might be across the world.