I think the main trouble is that to increase passing skill you MUST train passing.
It is no secondary in any training.
In other training rotations all interesting skills are in 1 maintraining, and in several secondary trainings, so when you rotate it all goes up gradually.
if you rotate PG training, it is as you say, you can better leave 1 main training out of the rotation, because it is as secondary in all the others, while for passing it's not. This creates a sort of imbalance which you try to right by skipping a maintraining on 1 skill, but it's not that smooth as a fluent rotationprogram.
Because you'd have to be more carefull how precicely to rotate your training to get a good effect, I think many choose for the easier "one-of-each-main" training...
In any case, somehow, for a decent PG, you'll have to pay a decent amount on the TL...
They are not your friends; they dispise you. I am the only one you can count on. Trust me.