Earlier quoted context omitted.
From what I have understood of this topic: If the relationship continues, the power could be used to favour the subordinate over other subordinates (give them good reviews, etc). If the relationship ends poorly, the power could be used to spite the subordinate over other subordinates (give them poor reviews, etc).
But really, how is that different from any other decision an employee might do that prioritises their own personal gain over their company’s benefit?
Companies that are large/sophisticated enough to have formal policies of employee conduct generally ban a wide range of behaviors that prioritizes the employee's personal gain over the company's.