The only official FAA purpose to log landings is for passenger currency. If you look at the landing currency reg, it very clearly says that in order to log a a landing for currency, a person must be the "sole manipulator of the controls" for the landing.
Period.
If you want to log students' landings for some other purpose (I'm not sure why, but for some reason some CFIs want to), differentiate them in some way so the FAA never has to question whether you were falsifying your logbook.