An acoustic guitar is a remarkable piece of engineering. It is at the same time quite fragile, yet also remarkably strong. The entire structure is built around the need to produce a good sound while resisting the force of approx 180 pounds (800N) of tension exerted from the strings which are trying to pull the head of the guitar around toward the body. When this tension succeeds physics has won the battle and the guitar then ranges from difficult through to impossible to play. This can take a long time to occur, sometimes years, and we only start to notice when it’s getting too late to save the guitar. Only an expensive guitar is worth repairing (if it is repairable) and it will need an expert guitar repairer, usually at great cost.

Extremes of temperature and humidity greatly aid nature in its quest to bend a guitar out of shape. The inside of an acoustic guitar is unsealed timber. Timber will absorb and release moisture according to the environment it lives in and the weather it is exposed to. Timber will swell a little when exposed to high temperature and it will shrink a little when exposed to cold temperature. Timber will also swell when exposed to high humidity and will shrink when exposed to low humidity. If you combine high temp with high humidity or low temperature and low humidity the results are compounded.

If a guitar has change in environment it will adjust to the change without a problem, it’s the repeated change in extremes that does the damage so it’s best to avoid extremes and repeated sudden changes of temperature and or humidity. If your guitar lives in an air-conditioned environment then the guitar will have low moisture content, if you take it into and hot moist atmosphere will cause the timber to swell rapidly and if the guitar is then returned to the air-conditioned room it will then loose the moisture again causing the timber to shrink again. If you do the opposite will have the same effect except in the reverse order. Also, do not to leave your guitar in a closed car in the sun, as this can raise the temp of the guitar to extreme heights.

A lot is said about keeping your guitar in its case to keep the guitar more stable. I’m not sure of the worth of this thinking as 1. The guitar case would have to be quite air-tight for this to work and 2. I would be very concerned with putting a guitar that has a high humidity content into a guitar case as this would help lock the moisture in thereby greatly increasing the risk of mould developing inside the guitar. Timber and mould do not get along together at all.

Just be sensible and you will get a number of decades out of a quality guitar.

First of all, why post this here?

Second of all, the reason so much is said about keeping the guitar in it's case is because the case will insulate the guitar from temperature changes and allow it to adjust to a new environment slowly, thus minimizing th damage to the guitar.

Locking in moisture is a good thing in most climates, as the outside humidity is too low. Excessive moisture is not locked in because as you said, the case is not air tight.