I am reading a book about the history of Star Trek at the moment. In the days of TOS in order to calm tensions between Nimoy and Shatner TPTB created a "Favoured Nations" clause in their contracts which basically means that whatever one of them gets the other one has to get the same.
This proceeded to the TOS movies and is the reason Shatner directed Star Trev V.
So does that mean we need to see Shatner in one of these new movies?
The "favored nations" thing was something they used to negotiate parity in their contracts for the Star Trek
movies. If Paramount had declined, for example, to let Shatner direct Star Trek 5
all the Shat could have done would have been refuse to participate in the project. There's no way he could extort anything from the studio beyond that.
Just because Abrams hired Nimoy to appear in a movie doesn't entitle Shatner to a job in another.