The producers of the new movie wanted a younger cast so the film would appeal to a younger audience. Leonard Nimoy played Old Spock ("Spock Prime" in the credits) as part of the reconciliation between the new timeline and the old timeline.
Leonard Nimoy's Spock is the symbol of all Trek; I guess they figured they needed him to reboot the franchise. I agree, because the message they wanted to convey was the original cast members endorsing Abrams' Star Trek ProfessorTrek 07:36, August 8, 2011 (UTC)
Somebody had to show Chris Pine how to act. Alas, even the great Spock couldn't accomplish the impossible. Admiral William Thomas Riker 01:12, October 16, 2011 (UTC)