Permissible or not ? I'm all resi, and just never paid any attention to this issue. Thanks
Yes absolutely receptacles are allowed above drop ceilings.
However you may not use them to feed a flexible cord, because 400.8 does not allow flexible cords above ceilings.
Low voltage transformers (wall warts) may be plugged directly into the receptacle if they are Class 2 devices and their output cord is not one of the NEC prohibited flexible cord styles. another use for these receptacles would be for portable/temporary equipment (i.e. work light)that is removed prior to "closing" the ceiling.