Hardly. For actually observing with a telescope UT1 is the correct time scale to start the calculation from, since it's directly linked with Earth's rotation -

Hardly. For actually observing with a telescope UT1 is the correct time scale to start the calculation from, since it's directly linked with Earth's rotation - with some complications that you have to calculate local sidereal time and so forth, but this only involves fixed constants; All TAI derived fixed-offset time scales are not linked with Earth's rotation and thus require constantly updated offsets. For most telescopes approximating it with UTC gives good enough results (pointing accuracy wise), so that's what many observatories do. And many smaller and older observatories operate quite a lot of legacy hardware and software that would need to be updated if the current UTC definition were to be changed.