As you say, hot and humid, is actually not what you want to have inside by opening up the boat. Not good for the woodwork, fabrics, carpets or perhaps artwork on the walls.
Air Condition is doing what it is named after, taking away the humidity and stabilize the temperature inside. Normally half of the air is from outside and the other half is recirculated so it will not be too dry or too damp, still oxygen enough for you to breath...
If the system is designed so you can shut down half of the compressors and run a smaller genset, which you normally have to anyway, the cost will not be too high...