-Kiln dried wood is not considered an option and only properly air dried wood is used
This is fine if the environment in which the constructed piece is to reside is of the same relative humidity as to which the wood was dried. In Japan, where central heating is quite uncommon, and there is a bit of a cultural idea that the house environment should somewhat reflect the outside environment (i.e., stifling/hot in the summer, drafty/cold in the winter), and traditional house construction revolved around maximizing ventilation/air circulation above all else, the relative humidity inside a house is going to largely harmonize with the relative humidity outside, so air dried wood will work fine.
In many other countries however, people do not like to, say, huddle around a charcoal brazier or under a heated table in the winter and prefer central heating and insulated houses. People in the US prefer to keep the house temp relatively consistent - warm in the winter and cool in the summer. Keeping a house centrally heated requires the addition of heat, either blown air or radiant, etc., and thus the relative humidity inside a house is generally going to be lower than what it is outside - or in the case of the area where i live, people will add humidification to certain rooms, like bedrooms, to keep them from getting too dry in the winter. Furniture inside a house is going to be exposed to much drier conditions than eventuate with seasonal air drying cycles.
If the furniture piece is to be subjected to life in a conditioned interior space, that is, a space which presents an artificially more or less dry environ than otherwise, then the wood must also be subjected to a drying process which brings the wood to a low enough moisture content to behave well in that situation, and that means exposure to greater heat and lower humidity than is present in the normal seasonal fluctuation.
The floor in our house is a great case in point. We have a super-insulated double-wall house and we use only a small heater in the main room to heat the entire house. We keep the temperature usually in the 64~66˚F zone in the winter, which is lower than many people do. The wood flooring in our house was locally sourced ash, air dried, and it does not do so well in the winter months. The floor shrinks quite a bit and nearly 1/8" gaps form between boards. It was a warranty claim issue for which we received compensation. When I replace the floor down the line, it will not be with air dried material, regardless of how long it has seasoned.
In general in a North American context, one can consider a typical seasonal relative humidity level
in a house to be 50%RH, and if you want wooden items to behave well at that RH level, then the wood needs to be dried to an equilibrium of 8~9%MC.
Some areas of the US, however, are much more challenging, like Salt Lake City, where the relative humidity is so low it is almost impossible to get wood dry enough - the 4~5% zone -that it does not shrink a bit after installation.
Air drying is an important step in the preparation of wood, and may be enough for certain situations.
However, I'll quote a passage now from William H. Brown's
The Conversion and Seasoning of Wood, from chapter 6 'Air Drying', where he's taking a look at conditions in the United Kingdom:
It will be seen that the average relative humidity throughout each twenty four hours is in excess of 80% for a large part of the year, equalling a drying equilibrium for wood of more than 17 percent, while the best drying periods are during the daylight hours from March until September, but taking an average for the whole year, relative humidity is 79.5 percent and equilibrium moisture content 16.4 percent, which is in effect about the driest one can expect air dried wood to achieve in this type of situation. The annual averages given are based on Meteorological Office data covering a ten year period, i.e., 1961~1970 and it will be seen that taking the British Isles as a whole, equilibrium moisture content values for wood stored in the open air are a lot higher than is generally realized. The summer months do provide lower equilibrium values and obviously have a bearing on the condition of wood, deemed to be air dry, if extracted from the yard during this time.