Did anyone really care whether or not Valeris betrayed Our Heroes?
It was definitely predictable if not obvious but I think it still worked, Nimoy and Cattrall established well that Valeris had been his protege with Spock already pleased and with a lot of hope for, then disappointment at her.
Granted I was a kid at the time, but until that "reveal" I thought that Valeris really was there to take over Spock's role as resident logician with whatever crew was going to be on the Enterprise-A in the 'next' movie. Because we were going to get a new group of heroes for even more post-TOS movies, right??
Even in general, the film does very much, pretty often try to be a bridge to a next generation, given that (and that the film on the whole is pretty feel-good) that the traitor was one of the main crew, and that transition character, is still somewhat surprising.