A common problem is found in B.C. when people pile lots of insulation into suspended ceilings below cold attics. Ventilating the space above the insulation to the outside can only be recommended if you have successfully made the suspended ceiling air-tight at the ceiling level. If anyone has invented a way of doing this on a suspended ceiling I sure would like to hear about it.
All I can recommend is do not insulate in the space above suspended ceilings -- unless you put Styrofoam directly on the uppermost ceiling with no air space behind it.
If you insist on putting fiberglass above a suspended ceiling anyway, try filling the entire cavity with insulation with no air space at all. The objective here is to block air movement and hence minimize how much moisture actually succeeds in reaching the cold original ceiling. The principle is quite similar to sealing off the flat roof -- and the results even more touchy because of the poor seal at the dropped ceiling level. Watch it carefully for moisture accumulation deep in the insulation.