The ability to emulate the State of Being and Source Origin of any subsumed entity was too good! It would allow Draco to wear and use the essence of other races that weren't included in his bloodline.
For example, if he managed to encounter a rare species with a powerful State of Being, he could subsume them and later 'become' one of them. Of course, this wouldn't give him their race or organic benefits, but was limited to their State of Being alone.
It was a similar situation to being granted Draconic Source Origin through the Dragon's Blessing potion. One would gain the Source Origin of a Low-Rank Dragon, which would allow them to move Worldly Energy like a Low-Rank Dragon, as well as possess some of the basic benefits of the Dragon race.
Chiefly, if they had children, such offspring would organically possess a Draconic Source Origin and even manifest a bloodline, though it would be very low in purity. This was why the potion had been so hot back then.