Systems of relative order zero in continuous time are strictly causal

The  ZOH discretization method does not map the continuous into the discrete poles according to the theoretical definition

Only one continuous signal can be obtained from a discrete time signal

Diverse discrete systems can approximate the operation of a continuous system

Signals of relative order zero in discrete time are common

The relative order of a transfer function is the order of its denominator polynomial

The root locus technique studies the stability, precision, speed and oscillations of a control system that adds a proportional controller and a feedback to the plant 

You can use the double Bode diagram to study the stationary state response of a linear stable system to periodic inputs  

If we can design a stable system then we can ask that it must be useful