What an excellent thread, enjoyable read and the pdf document link is excellent too.
I'm part-way through an FI course. The turnback was a demo and practise on my pre-course entry 'test'; and it's been done once more at the end of one of my air exercises both at height, and in the circuit.
My FI instructor wants me to experience the turnback for two reasons: to sharpen my handling skills; and so that I can talk from a position of strength to my students to give weight to what will be my advice of 'don't try it'. Fair enough.
It'll never, ever be used by a PPL with any success. The vast majority I'm sure won't have the handling skills, or presence of mind to do one successfully. More to the point there's no performance data in the flight manuals regarding turnbacks; there's nothing like the CAA Safety Sense leaflets for turnback guidance (these UK CAA leaflets are great for giving you factors to apply in a variety of take off and landing scenarios) which would give info such as increase airspeed by 'x' or never exceed bank angle 'y' or perhaps most importantly, what to do in wind. The above pdf is great for aero engineers like myself or Genghis, or tp's or the very rare flying techno-anorak. But a ppl won't go anywhere near it - nor will most FI's.
The turnback is all-or-nothing. If you don't get back to the field with it you're arguably in a much worse position than a crash straight ahead. Ground speed in any sort of wind will be greater. Eg in a 10 knot wind I'd rather crash straight ahead at a groundspeed of 30 knots (I fly a DHC-1) than turnback, fail to get to the field, but crash at 50 knots -