The danger of the shift in ruling philosophies that was seen in the reign of Xerxes I was that that shift would bring about more rebellions and tensions within the empire. This danger actually was realized in the rebellions in such places as Egypt and Babylon.
The shift in ruling philosophy was a shift from toleration to a lack thereof. Xerxes’ predecessors had been religiously tolerant. They had held their own religious beliefs, but they had not tried to impose those beliefs on others. Xerxes broke away from this tradition. He tried to force various peoples of his empire to adhere to the same beliefs that he held.
This led to rebellions because it alienated the populations whose religions he tried to suppress. For example, Xerxes angered the Egyptians when he refused to be crowned as their ruler using the rituals required by their religion. This did not seem like a major problem until the rebelled. Their rebellion eventually led to them breaking away from the empire.
Toleration tends to make empires more stable. When Xerxes chose to move away from toleration, he set in motion forces that weakened his empire. This was why his shift in ruling philosophies was dangerous.