It makes perfect sense for Meyer not to use Curtis -- Saavik was shown quite emotional as a "Half-Romulan" in TWOK. And while Catrall didn't play Valeris nearly as emotional as that -- she was far more emotive than the way Curtis played the character in TSFS. I think the Saavik/Valeris character in VI was a more under control but still with a lot of turmoil under the surface to have been part of such an extreme plan. I couldn't see a full Vulcan justifying murder, treason, conspiracy, war, betrayal -- but the conflicted half-Romulan whose adoptive family (Enterprise crew) had been thru so much -- the loss of her fellow cadets, Spock (temporarily), the Enterprise, David, etc.
Combine that with the way the crew was treated by Starfleet -- may have led her to rationalize her actions. Curtis' Saavik just didn't seem to have the energy for anything like that.
Combine that with the way the crew was treated by Starfleet -- may have led her to rationalize her actions. Curtis' Saavik just didn't seem to have the energy for anything like that.