There are some exceptions such as railcard users travelling at times where there is no minimum fare (as mentioned), cyclists who wish to travel with their bike, people with luggage and the less physically able who would find an additional change an inconvience.
Yes there is no railcard discount so railcard holders should get the Manchester Central Zone ticket and go via Stockport. However, despite what was said by MCRUA in the quote above, I'm not convinced travel via Stockport isn't allowed on a Metrolink City ticket. based on what it says in the FRPP. FRPP only says you can travel via Sale with a Metrolink City ticket. It doesn't explicitly ban the Stockport route and the route on the ticket is 'route Altrincham', which is on the way from Knutsford to Stockport.
I reiterate my question: can anyone put forward a coherent argument why access to the FRPP should be withheld from the general public?