Independent support really all depends on which seismic zone one is in.
http://www.designco.net/us-seismic-zones.gifIn California we're required to support the opposing corners of a troffer fixture to the structure above with wires, and screw the remaining corners to the grid.
We are required in most places around here, to attach some kind of additional support, usually ceiling wires, from the structural ceiling to all lighting fixtures that are installed in a grid ceiling.
In Pennsylvania, the State is split between two seismic zones. That could be the reason.
