I think having a unified time everywhere is a really obvious advantage. Time being measured differently based on geographic location is completely silly. You would still need to look up how a different location is scheduled but you also need to do that with timezones so there's no disadvantage, only an advantage.
It's a matter of compatibility, e.g. you're not going to start measuring length in meters when everyone else in your country is using feet. That's why I said UTC is a time zone, nobody uses it outside of the UTC time zone, therefore nobody uses it outside of the UTC time zone.
