"Leslie
I agree that building the "leaning rectangle of speed" is the best approach to driver training. However I think the best path to that end comes from looking at the places that defensive, mental, and attacking have crossover in the attributes that they train.
Both Defense and Attacking train the attribute Decisions
For the sake of this analysis I will only note that Mental trains Bravery and Focus which are also trained by attacking and defense.
In order to build the perfect rectangle it is probably best to avoid crossover training in these attributes. For example training Defense slightly ahead of Attacking will avoid the crossover of decisions.
But even these things are not my primary point.
The attacking attributes of aggression, bravery, and overtaking are just as important to race pace as the defensive characteristic of focus. Especially in the middle to later stages of training your driver. Thus avoiding Attacking training will inhibit your drivers overall speed.
Also the Mental characteristics of anticipation and composure are important to overall race pace.
I have seen my drivers pace seemingly jump by training neglected attacking. I have also seen the same thing by training neglected mental
Define 'rectangle of speed'
also, attacking does NOT train decisions.
What most people seem to do, and I think is best to train speed, phycial, defence AND attacking but you have a free choise of what to train so if you think some thigs are good to train you might gain an advantage on most drivers :D