It's a nonsense argument against a unified time zone. It's pretty much guaranteed to happen sooner or later, it just makes sense as globalization progresses, communication is instant and global and the world grows closer and closer.
"I'm in Berlin, my working hours are 7:00 UTC to 15:00 UTC, and I'll be reachable until 20:00 UTC".
Arguably, without timezones, it's much harder to figure out when its daylight in a particular region, so when someone should be reachable.
"Well it's 06:40 UTC, but what time does the sun rise in New York? Has the sun set in Bangkok?". Meanwhile having googled that it's 02:40 AM in NY means I can be assume a normal-working person would be asleep at this time.
Perhaps that's cultural. Most people in my social circles care quite a bit about sunlight. At home, the evening routines that we do with our children are based on a combination of season and sunrise/sunset. I find it somewhat surprising in fact that this is not at least partly the case for others, excepting of course people that work 3-11 or 11-7 or what have you.