Hi Kristina,
It is just there as an additional option.
Some people use only motion derivatives, some only motion
parameters, some use both. The option was added to allow
capturing more of the motion effect in the case of resting state
analysis, but it does not necessarily produce a big gain. It is
less of a question in task-based FMRI.
In any case, we do not say that everyone should be using it.
It is simply available.
- rick