It would have changed everything of the story since the beginning because being so powerful they would have ended every fight in one second.
The luscious nyan cat takes her stand against the sultry May the best girl win! Akeno is seen as Rias' main rival for Issei's affection, as both are the most seductive towards him, and their fights will sometimes cause the other girls to back off.
Akeno also has the power to purify evil spirits which she learned from her mother.
The orphan wielding Aura's Storm is liked by both women.
For the same reasons why never saw Maous or Rating game players help in the fights.
Akeno cares for Rias and does not want to destroy her relationship with Issei, but she does declare that she would be Issei's mistress.