I have a 45 year old Yamaha D-60 classical guitar. It has traveled with me from Death Valley to Kwajalein Atol. I've played in in Colorado and in Cancoon. It has lived in the Desert West of the US and in Germany. It has traveled by plane, boat, car, bicycle, and even my tank. It has survived prolonged exposure to <5% relative humidity as well as weeks above 80% Rh. The neck is true, the finish is flawless (except where I dropped a 3.5 inch antitank shell on it producing a divit on the front). It sounds better now than it did when I walked out of the store with it new.
I have never kept this guitar in a 'humidity controlled' case. Could it be that a cheap Yamaha guitar is better built than a Martin or a Seagul that seems to break when they get dry? The truth about wood and humidity is that the wood needs to be assembled properly so that the grain of the wood shrinks or expands evenly and appropriately. Any good woodworker (hopefully any competent luthier) understands wood grain and how to properly join wood. If a change in relative humidity causes your guitar to crack at a joint or to delaminate at the bindings you are dealing with poor craftsmanship and need to look at another manufacturer for your next guitar.
Of course, if you have a poorly designed and assembled guitar (regardless of what you paid for it), I guess you might need to keep a sponge in the case with it to keep it from going bad on you.