Blocking the rudder isn't for the student's benefit; it's for yours. It's a safety issue. The student really ought to be able to do partial panel at night in turbulence, but one need not wait for those conditions to put them through training or a practical test, and one need not let the student go to a full Vmc departure without any reserve rudder available, either.As a check airman, CFI, examiner, FAA Inspector, I never blocked the rudder for a demo or checkride.
Blocking the rudder is a safety technique, and a valid one. It robs the student of nothing, and it can be used to enhance training. In an airplane that won't reach a departure until very close to or below a stall, blocking a rudder ensures a departure or signs thereof at a higher speed, while still maintaining safety. It ensures the student gets to see, feel, and demonstrate the loss of directional control as well as the recovery in a positive and safe manner.
It's prudent and proper.