Just read a great paragraph on why having all ages and levels of experience in an orchestra is so important, and why being ignorant of this is simply to be stupid:
Like fibers of a rope, not a single one of which runs the entire length, the overlapping career spans of musicians carry on the traditions of an orchestra. An orchestra without this linkage to the past isn’t really an orchestra in the way we currently think of one, it is merely a group of musicians, a pickup group. The attitudes of managers and boards of directors in some places, where they assume musicians are easily replaceable from the stocks of eager conservatory graduates (who, most importantly, would work for less money) are misguided at best, destructive, and not in the interest of the institutions they serve. Cut too many fibers, and the rope frays and breaks; braid in too many, and it becomes thick and inflexible.
From Bass Blog