I take your point, and Reinier had also expressed the same view, however doesn't turning the foot to the back lead to a less stable lunge, as well as introducing a rotating motion from the back foot in contrast to a straight piston-like push from the entire back leg?
Recovery also seems slower since either the "pulling-back" action of the back foot is done with only the ball of the foot, instead of the entire foot, or while moving back into guard the back foot needs to be repositioned.
Performing the lunge and using the ball of the back foot to push backwards also feels like a groin strain waiting to happen, but note that these observations are my own and could be a result of my lack of experience.
Next, Reinier, I'm afraid I've not read Bruchius, but we can talk about the others
In Giganti you are right, I just flipped through and missed the back foot positions in plates 4, 5, and almost all the plates with dagger. If you look at plate 11 there is even a rolled ankle, not to mention plate 24 etc with a raised heel.
Fabris' illustration are slightly ambiguous, although that could be due to the perspective. Then again this is Fabris, and I'm sure he has a backflip killing move somewhere in book 2
Agreed that Capoferro doesn't show anything like that, although I wonder if the 'raised heel' is due to perspective.