I've spent literally hundreds of hours thinking about this, and I've landed on the opinion that TAI should always be the source of truth. It's usually not, because most systems ultimately get their source of truth from the GPS system, which uses something that looks kind-of like UTC (but is actually more similar to TAI, but people prefer to dress it up as UTC). This was a reasonable mistake that has caused so many problems due to the leap seconds.
If you're using TAI, it's more obvious that you have to account for leap seconds in order to convert to a human-readable civil time format (year, month, day, hour, minute, second). You know that you have to incorporate the official leap second list from IANA or some other authority. UTC makes it seem simpler than it actually is, and that's why there are so many problems.
You can always convert from TAI to civil time. You cannot always go the other way.
If you're using TAI, it's more obvious that you have to account for leap seconds in order to convert to a human-readable civil time format (year, month, day, hour, minute, second). You know that you have to incorporate the official leap second list from IANA or some other authority. UTC makes it seem simpler than it actually is, and that's why there are so many problems.
You can always convert from TAI to civil time. You cannot always go the other way.