Denny wrote:

Another interesting point is that many private aircraft don't stand up
to being thrashed daily as a trainer... I have noticed in the past that
PIC has mentioned that a significant percentage of private owners who
do the 10 day instrument course have to rent a plane to finish the
course as their personal airplane didn't make it to the finish line...

I don't see why rental aircraft should be anything special in this situation.
What you're saying is that many privately owned aircraft can't fly for 50 hours
or so without something breaking. If that's true (and it's certainly not true of
mine), then many rental aircraft can't do it either. The only difference would
be that the FBO usually can provide another aircraft if a rental breaks down.

As I understand the situation, the main reason that pilots don't complete an
instrument course in their own aircraft is not because their aircraft breaks
down but because their aircraft lacks some of the equipment necessary for the
checkride. They finish up what training they can in their plane and complete the
training in an aircraft with better avionics. This was especially true back when
an ADF was required for the checkride.

George Patterson
The desire for safety stands against every great and noble enterprise.