If it's not the default clean behavior it is useless in my opinion. Make clean is called on a regular basis as part of the development process (due to changing boards and flags), so I'd have to download the same sources all the time.
If I'd want to "reset" the port for other reasons, I'd have to manually make this target as there is no clean target in RIOT (and it's function is already present in clean for the rest of RIOT). In that case I could just as well run the git clean command manually.