Kirstie Alley created a fascinating new character in a familiar (if beloved) universe. From what I remember, she made quite an impact even on people who were not regular Trek fans: she looked and acted the part so beautifully. It was a great letdown to find her missing from the next movie, and to have the character played by someone who seemed to be just a generic Vulcan. It should never have been recast.
Alley asked for quite a bit of money to do ST III - a bit of a deliberate deal-breaker: the other actors made the work experience so unpleasant on TWOK that either she was going to get paid an awful lot to repeat it, or she wasn't gonna do it. Frankly, her failure to play Saavik again was scarcely a huge missed opportunity or a blow to the career that she actually has managed - where's Robin Curtis these days?