I'd wait to see what was actually meant by this before condemning this - HOI2 didn't have divisional leaders, at least in as much as there was no leader slot for every division but instead only for every stack.
This isn't strictly true. Back when the AI randomly choosing who to autopromote was the only way to raise a rank without losing skill, making optimal stacks wasn't possible.
So for China, who started out with 36 divisions, making stacks of 3 or 9 would mean that at most a dozen generals would be leveling and they'd have to do it slowly since they'd all have to be ranked high enough to lead a stack of 3, 6 or 12.
But the stacking limit applies to units in combat not units in a single stack.
So for China, which has crap field marshals and generals, it's much better to have every division be a single division and stack none of them.
This way, instead of having Chiang Kai shek leading a stack of 12 and gaining experience slowly, you can have Chiang Kai shek leading 1 division, gaining experience slowly, while 11 other divisions are led by 11 major generals, all of whom level up very quickly. As long as Chiang Kai Shek is in combat his stacking limit of 12 applies to everyone and now you have 12 generals gaining experience where a traditional stack would only have 1-3 generals.