The turn back has always been a regular discussion. One of the best sensible discussions I heard was at an Instructor seminar by on track.
Points brought up.
1. RAF used to train the turn back but had more aircraft lost due to training for it than actually it saved. This was with highly experenced QFI's instructing.
2. The height which you can turn back successfully is related to aircraft performance.
What a super cub can get away with is completely different to say your average flying school heap.
And some of the motor glider types it was said from 500ft they could do a full circuit.
3. There are many variables which will alter at what height you will be successful from.
4. Luck plays a huge factor.
Now personally if the donk fails the aircraft has now become the property of the insurance company. The air frame is disposable, I will use it in any manner that will minimise any damage to the soft bits of meat that it contains. Personally I wouldn't consider turning back as an option until I had started turning crosswind.
As with all emergency's if you have a cunning plan of what you would do things generally are more successful if you stick to it. So if you have planned to always go straight ahead and possibly where you have the best fields you will 99% of the time have a better outcome than if you make something up on the fly.