Is there any more reason to worry about the effect of humidity on wooden musical instruments than the effect on wooden furniture?
Humidifiers and dehumidifiers for use in cases probably can only provide for grams, not ounces, of water, and probably are only "effective" for a few hours.
Museums storing historic instruments for centuries may be a special case. Even there, conditions within display cases will change.
Good cases are valuable. They keep off rain, dust, splashed paint and small fingers, and minimize damages when dropped.
