Super65,
I wouldn't use a humidistat with it becuase I would never trust it to stay in calibration for calculating the dew point for controlling the cooling coil valve. If I were going to go to dew point control, I'd probably be looking at a chilled beam first. In most applications, it suffices to have a cooling coil temperature below the dew point 99% of the time. Where I'm at, that's about 54* F. I would include a humidistat for humidification.
I'm not sure what the connection is between calculating minimum air flows for indoor environmental quality and latent loading, and it has no connection to thermal loading calcultions. For example, if limiting discharge temperature as per ASHRAE 55, using max 95* F supply air, and 4 ACH will not provide enough heating, the only thing to do is increase your supply air. If you already ordered and installed your AHU based on 4 ACH, you are SOL. Or, you might reset the discharge temperature to about 135* F, and hope no one notices until you're out of town.
Answering the question in the OP, there is no logic in the rule of thumb. Sort of like "I need to hook up 480V from the MCC to the fan motor, why can't I use thuis spare lighting cable?" You will find more logic in watching old episodes of Star Trek with lots of Vulcans. Repeating what is often said in the threads, recommend you get a mechanical engineer to do mechanical work. If this is some sort of cross training thing, maybe you could hire a mechanical to do the electrical design, then he/she can pose their questions on an electrical forum.