Yup, i guess those are the main reasons. Combining derate and ATM allows reduction of quite a bit more than the usual 30% that is available with ATM alone.
And of course, derate has some merits, especially on contaminated runways where it might allow to carry a higher weight of the field than a full thrust (no ATM allowed) would be able to. The lower VMCG and thus lower V1 make it possible. It has some problems to as it is usually not allowed to go to full unreduced thrust in case of an engine failure as the resulting thrust might lead to loss of control.