I wrote to GH and asked them why they stop using it after veg. Here is what they said:

"The Diamond Nectar aids in nutrient absorption and mobilization. In vegetative
growth, that means a healthier plant with access to more nutrition. A side effect
of this is a decreased length of your vegetative growth phase, which is why we don't
recommend it in flowering, as it will affect the way the plants feed and possibly
not allow full maturation."
Makes sense!!! Similar to one of the reasons organics tastes so good. Takes longer, giving the chemicals in the plant time to do there thing... Growing is all a patience game...