I have a suspicion that there's actually not much difference between save and evict. I mean, if this year had vote to evict, could we reasonably still expect Gemma or Dion to have lasted much longer than they did?
The worst thing with vote to evict is when boring housemates stay in the game because everyone becomes obsessed with voting out the controversial characters. Vote to save isn't perfect, but it seems a better choice for keeping an interesting, entertaining mix of housemates in for longer.