Despite twenty three years of maintenance in Borneo where the ambient temperature is a fairly steady 32 degrees celsius and the typical relative humidity value lies between 90 and 100 per cent I have never known any avionics equipment to be a source of water such as Milt describes. Perhaps he is referring to military aircraft ? The operating conditions in the cabin of a typical airliner are very dry, with relative humidity values below 40 per cent. The equipment racks are within the pressurised zone of the aircraft and share the same controlled temperature and low humidity environment. On the ground, the air conditioning system removes water in the water seperators, keeping the humidity down to comfortable levels. In the rare event that an aircraft remains on the ground without air conditioning for any length of time, the R.H in the aircraft rises to match the ambient levels but the relatively high equipment temperature prevents any condensation in the equipment racks.
I don't believe that de-humidifiers are justified in any airliner that I know of, in fact the opposite is true. There is a strong case for installing humidifiers for passenger comfort reasons.
There is however a strong case for storing electronic equipment in non-airconditioned but de-humidified storage conditions. Here in the tropics, it is very common to experience heavy condensation in avionic equipment just drawn from storage.