I am curious to know why we can't apply control algorithm like PID on the weighted signal of Roll and Roll rate in a quad-copter instead of using two loops to control them independently. Fundamentally PID will make the input signal to approach $0$. In the case of (Roll + Roll-Rate), which would be $ w_1\theta + w_2\dot\theta$, the sum becomes $0$ when individually both tend to $0$; since we have an exponentially decaying curve.
So why do people generally use sequential loops to control each of them separately? (Roll is just for example)