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.

As long as someone wants me to provide a receptacle above the ceiling for those purposes, I'm fine with it.
But as soon as they say that they want it to plug something into it and leave it (like a condensate return pump), I get antsy. I don't want to be part of their violation of 400.8; I wish that the Code was worded differently...it seems to be trying to fix one problem by addressing a different issue.