All the answer except for quiet flyer's are spot on. Although, I do appreciate, acknowledge and understand his point of view. 90 years of instrument flight has dictated which instrument is primary and which is secondary for each state of flight. It is not arbitrary. The way to look at it is that you are trying to achieve a certain outcome without the benefit of your usual feedback. You can not rely on on sight, sound (in some cases), touch (seat of the pants), or sense of motion and balance.
Say for instance, if you are flying IMC. And, you want to fly straight and level in unaccelerated flight. The Airspeed Indicator is your primary for power because (hyperbolically) who cares how fast your prop is turning. Your RPMs are not an outcome. At least, it is not your desired outcome. Your desired outcome is a constant airspeed. Your engine performance is tangential to that. In fact, your RPMs are going to change as your Density Altitude changes even at the same airspeed. Then, throw into the mix a constant speed or otherwise adjustable prop, your Tach becomes almost unusable as a primary for power except when you are changing from one maneuver state to another.
Personally, I think that the Control and Performance Method is much better at addressing all of the posters' views. Especially quiet flyer's. In reality, everyone that I fly with uses Control and Performance intuitively. In VMC, Control and Performance is automatic and second nature because you are using sight, sound, and sense of motion primarily to fly the plane. It can still be used when those senses are not available in IMC.
Where the Primary and Supporting Method shines is when you start losing the use of your instruments in partial panel flight. It tells and teaches you what instruments (supporting instruments) can be used when the ones you want (primary instruments) are not available or inop.