From my own experience I would say they are quite necessary if you are involved in ultra long haul flying. If you know anyone who was crew on the original 747SP ask them what they thought of the long sectors, there were several reported cases of illness attributed to the lack of humidity at high altitude for long periods.
Wouldn't hurt to get your union, or representative body, to ask engineering for chapter and verse on this one and see if there is an alternative to just switching the system off.