Trek movies already have a rough time fitting all the characters in and giving them meaningful things to do. Spam a movie with all the crews they can afford? It'll be the Picard/Janeway movie with a little Archer and Riker thrown in, and a shedload of uniformed background people squeezing in a catchphrase or two.
Hey, if X-Men: Days of Future's Past
turns out to be the biggest thing EVAH, someone at Paramount might want Trek to do something similar with the surviving TOS crew and their young incarnations... but I seriously doubt it.