NEC requires lay-in's to be secured to the grid; clips, screws, rivets, etc. That is all!
Mfg specs (For some ceilings) may require support; Article 110.3 (B) says we must follow mfg spec.
Seismic code (Building) requires independent support.
Some Fire Inspectors/Codes require independent support.
The "Rule of Common Sense" which is NOT in the NEC leans to 'if the ceiling grid sags from the weight of the fixtures, independent support required' but that's not NEC
Ask your local inspector what article he is referencing.
