In a grand, the system primarily protects the soundboard, and area immediately adjacent to it. It does not offer significant climate stabilization to the action. (it does in an upright, since all components are in a more enclosed area)

Using a room humidifier can help to moderate the dramatic swings in RH that we experience here in the northeast.

If you have forced air heat, please install a furnace humidifier. They are not very expensive and they work great if maintained. If you have radiator type heat - turn it off in the piano room if you can, if not, keep the piano away from it. Use a room humidifier, but keep it a few feet away from the piano. Try not to over heat the room - 68 degrees max - colder rooms retain more humidity. If you have radiant floor heat....God help you... That's the worst kind of heat for a grand because it radiates right up to the soundboard. Might as well build a fire under the piano.

When you refill your Dampp-Chaser with a gallon of water every ten days or so in the winter, it goes into the air in the entire room, not just under the soundboard.

While the benefit to the action cavity of a grand is not as great as to the soundboard, neverthe less the moisture gets distributed. Pianos aren't airtight. If you've ever seen one with smoke damage, you'll be convinced that air gets everywhere.

Cracks in the soundboard aren't the worst damage, in my opinion. The worst is when the pinblock loses its grip on the tuning pins, and the piano can't be tuned to standard pitch. Probably next worst is bridge damage, where the side pressure on the pins splits the wood. Both of these are expensive fixes (although there are short-term Band-Aids).

Humidity cycling causes glue joints to break, since cured glue doesn't change dimension with humidity swings, and so the joints come apart: cabinet, action, lots of places. Parts that move in close proximity to other parts bind up and stick (like keys).

Lastly, more stability of pitch means less wear and tear on the pinblock during tuning, extending its life.

Controlling humidity is like a seat belt: it can only prevent damage, not repair it. It's cheap insurance.

--Cy--