Not a very good fire drill ! A panic turning off of the master switch before any investigation has taken place followed by the discharge of the extinguisher.
Oh , and then turn the whole lot back on for the distress call and to restart the fire !..........I hope the extinguisher is big enough to put out the fire twice?
Who publishes this stuff ?
In the PA28, switching off the master switch would not immediately compromise aircraft controllability or safety. In instrument conditions suction gyros would remain functional and pilots flying at night would most likely carry a torch for standby illumination of the panel. If the system causing the problem can be isolated then the option of switching the master switch back on is restored.
As in all emergency situations, there will be judgement calls to be made following risk assessment. For example, is the fire severe enough to warrant discharge of the extinguisher, which will bring visibility and breathing problems (as will the combustion products of a fire which is not extinguished). The benefit of a distress call is that the emergency services will be alerted. You may well need them later, particularly if your landing is away from an airfield.
A final point is that checklists can never replace sound judgement. They are designed to assist you in dealing with most normal and non-normal situations. Occasions may arise where you need to deviate from published checklist procedures because circumstances demand it.