TAS can be used by ATC for the purpose of planning IFR separation,
because aircraft in the general vacinity of each other will have about
the same wind aloft.

Even with perfect wind aloft forecast, an aircraft on a very long
flight will have a planned average groundspeed that's based on wind
aloft along many sectors, and wind might be very different in each
sector. Such groundspeed can not be directly compared with the planned
average groundspeed on another aircraft on a much shorter flight along
the same route.

Grandmaster wrote:

Also, I have only been flying for about 4 years, 3 instrument rated but
have had an idea: doesn't is seem that it would be more advantagous,
for atc, to use calculated ground speed instead of TAS when filing?
Expecially in this age of not-complete but prolific radar coverage and
pretty accurate winds aloft models? Just a thought.