"William Shatner was a mediocre actor, who went on to play Captain of the Enterprise; but that was another lifetime." -Nero
Oh how soon they forget ...

Shatner made a huge mark on the franchise, but now his time is over.
To me its not a matter of whether Shatner should be in Star Trek, but whether his incarnation can make both a worthwhile and believable contribution. I doubt that would be any easier to achieve in the next film than the last.