For fans, I think SENS_ANGULAR_VELOCITY is a better choice than SENS_FREQUENCY.
I do understand the point that you are raising. It would be nice to have the commonly used units like RPM and Percentage available directly in RDM. The problem you run into is that there are an unlimited number of unit combinations.
Lets use fluid flow rates as an example (which could be applied to fans and fog machines), offhand I can think of at least 8 units that are commonly used:
Gallons/Hour, Gallons/Min, Cubic Feet/Min, CC/Sec, Cubic Meters/Sec, Liters/Minute, Barrels/Day, Gallons/Day.
Which of these should the standard support? You'll never find a superset of units that will please everyone.
By sticking strictly to MKS SI units the standard ensures that there is a simple, welldefined set of units that everyone can understand. There is one unit for everything. A controller can offer a rightclick menu with the option to convert to arbitrary units if desired.
I want a unit for furlongs/fortnight!
