Good points. Spock's character certainly evolves over the course of the movies. And if ever I came up with a plot where an older, wiser, more balanced Spock fit the needs of the story rather than the TV version, that would definitely be a "logical" reason to set the book during the movie era.
It all depends on what kind of story you're trying to tell. Rings of Time, for instance, was always meant to be a twisty time-travel adventure focusing on Col. Shaun Christopher and the first manned Saturn mission, not Kirk's midlife issues or Spock's emotional growth, so tying it into the movie era would have been distracting and irrelevant. IMHO.
As for Weight of Worlds . . . well, I was coming of a TOS rewatch when I wrote that book so I really wanted to capture the feel of the original series, while also giving Sulu and Uhura more to do.