The future of Star Trek on the big screen is a big question and at this point, nobody is quite sure what will happen with it. The reboot franchise produced by J.J. Abrams has been successful overall, and many fans and critics have been impressed with the way the films found a way to tell new stories with these classic characters. However, the original Captain Kirk does take issue with one element of those movies, the use of Leonard Nimoy and Spock. Shatner calls the decision to include the original Spock in the new movies "gratuitous."
The concept of the 2009 Star Trek movie is that, due to time travel interfering with an established timeline, a new timeline is created, causing the classic Star Trek crew of Kirk, Spock, McCoy, and the rest to go in a different direction. Leonard Nimoy appears in the first two movies as "Old Spock" who's there to explain what's going on to the characters, and also the audience. William Shatner recently told Express that he didn't care for the way Spock was included in the new films, saying...
Leonard Nimoy was in some of those films, but it was totally gratuitous. They just wanted to put Spock in there and I didn’t admire that.
While it's been widely reported that there was some tension between William Shatner and Leonard Nimoy at various points in their careers, it should be made clear that it appears from the statement that Shatner's issue here is the inclusion of the character, not the appearance of Nimoy specifically. Although, the quote is largely without context, so exactly what led to Shatner making the statement we don't know.
Leonard Nimoy's character was a significant presence in Star Trek, and he had a brief cameo in Star Trek Into Darkness, a film that would be the actor's final on-screen role. The death of "Old Spock" would be referenced in the third film, Star Trek Beyond.
For what it's worth, William Shatner says that he would consider returning to Star Trek if it "functioned as a point of the movie," which seems to indicate his issue. He simply felt that Spock's appearance wasn't important enough. And that's certainly true, the plot of the film was not about Nimoy's character. It was about the new cast of characters.
Technically speaking, I suppose Leonard Nimoy's appearance in the 2009 film was not strictly necessary. The project could have been handled without him. But for a lot of fans the inclusion of Spock was what made the whole thing work.
Fans of the larger franchise appreciated the idea that the reboot franchise made a point to say that it wasn't replacing the original series or movies, just taking things in a new direction. And the fact that an original series star was the one making that clear gave a sort of stamp of approval.