If we change our breathing rate so easily, then what is the point of measuring it?
We may be able to voluntarily control our respiratory rate, but only for a short while. The buildup or depletion of CO2 will force us, against our will, to either speed up our respiration or slow it down. So, you can't effectively "fake" your respiratory rate for long, nor, to an astute observer, convincingly.

Normal breathing patterns are "normal" for a reason. Aberrations from normal do occur in pathological states, but they are compensatory mechanisms driven by a number of factors your blood chemistries: the pH and the intimately linked CO2/H-CO3 concentrations, etc. You can pretend any way you like: you won't be able to keep it up comfortably. Also, normal breathing isn't simply "breathe in/breathe out". It is a more complex thing characterized by an inspiratory phase of certain duration, an expiratory phase of a different relative duration, and a normal, periodic deeper breath (close to a sigh), all done physiologically and unconsciously. 

With certain illnesses come certain breathing patterns. You might be able to fool one person's quick assessment, but not more than that. If the faked breathing pattern (largely unsustainable anyway) doesn't appear physiological, you will be quickly found out.

For that reason, a sustained increased or decrease in respiratory rate is indicative of particular pathologies, whereas a normal respiratory rate is a helpful sign.