I believe your suggestion not to shut off the main valve is inaccurate, not well researched, and could create a false sense of security for some. Any problem that might be caused by the absence of a humidifier, (In listening to your program for years I have heard far more talk about dehumidification rather than humidification), which is why you would leave the main on, would be far less than the damage that might be caused by a power failure in inclement weather which could conceivably cause some pipes to freeze and burst regardless of what has been shut off in the house.
I was not surprised to receive an enquiry such as yours. My concern with a humidifier (i.e. Flow through) is that many homes have major hardwood flooring and also furniture that requires a control on relative humidity. Otherwise shrinking in the winter months may cause floor squeaks and loosening of furniture joints.