some airlines that had purchased the option deactivated them...and some humidifiers created more problems than they solved:
http://www.pprune.org/tech-log/26240...-aircraft.html
edit: another interesting blast from the past on this issue, found at spectators balcony
http://www.pprune.org/spectators-bal...r-quality.html
newer optional cockpit and crew rest humifidiers, such as on A380, B787 seem to be based on different technology (evaporative cooling)
and if someone wants to ask why airlines consider paying for humidifiers in cockpits, crew rest, and first class cabins, it's because humidity there is lower than in other areas where the density of bipedal humidifiers is higher.