I don't understand the logic to this other than to prolong the war.
Basically ATACMS was provided in response to North Korean and Iranian BMs being supplied to Russia (ignoring the obvious nonsense about "warning Russia not to use long range missiles" because Iskander and all the other missiles Russia already used for a long time)
What's 100 missiles going to do? Russia has already used many more times that and Ukraine is still standing, so why would the calculus change on the other side?
ATACMS will probably work nicely against the supposed troops accumulations to open another front in Sumy/Kharkiv regions. But like you say it will prolong the war and be able to unlock more PR stunts...