Unfortunately, Valeris did cause one problem with the plot.
There could be a major traitor on the Enterprise
Is it :-
A - A character we know?
B - A character we haven't seen before this movie?
Having Saavik as the traitor would have been a bold choice, and would have made Spock's fury more justified IMHO. It would have also needed the character to have to explain why betraying them would have been logical - Valeris had a get-out as, apart from Spock, she hadn't even met anyone else before.