Doesn't the answer to this depend entirely upon individual circumstances? Yes, I can imagine that people in (say) the Mid-West or SW USA would need humidifiers. But I live in England where we do not have really extremes of temperature or humidity. In 40 years I have lived with the maxim that 'if you are comfortable, then so are your guitars'. We don't have aircon, or have the heat turned up high; and my guitars seem to like it that way. Some people in some parts of the world might NEED humidifiers, but I do not think that they are an absolute must-have for everyone.