I've been amused to see how many different aspects of Nimoy's voice have been mentioned in this thread. I was originally talking simply about PITCH, but you all have brought up some interesting points.
Although it does seem to me that in his two most recent, post-retirement screen appearances in the Abrams films, he hasn't really been playing Spock at all; he's just come in and been Leonard Nimoy.
Mr. Nimoy has said that he was intentionally relaxed as Spock Prime because he thought that by the time he was 157, Spock would have been relaxed and self-accepting and would have made peace between his human and Vulcan sides. So Nimoy wasn't phoning in his performance; he was intentionally taking the character to a new place.
I don't know how Nimoy's increasingly worse portrayal of the Spock character qualifies as "evolution." After STAR TREK II, Nimoy stopped putting any real effort into his performances.
You might want to read Mr. Nimoy's second autobiography, which includes the movie years; it is not true that he stopped putting any effort into his performances. Mr. Nimoy has thought deeply about Spock, and he has tried to give us a realistic character in spite of bad scripts, poor direction, and other impediments.