It was in a handbook i had in my last car.Something to do with the tyre width & putting power down & braking etc. 
....I haven't seen it in any handbook but agree that a space-saver is going to be far safer on the rear axle for the reasons you state.
I have no choice anyway because a space-saver won't fit over my AP big brakes. This will apply to everyone else with a big brake kit.
