Nimoy was just able to massage the egos of the other actors to keep them happy, not expand their roles.
Well, what would you rather do? Spend three months sitting in the background pushing buttons and getting paid for it,
having that one day where the focus of the director and film crew is on you
and you get to really sink your teeth into some lines that require comic timing, and will be remembered by fans at conventions forever.
Some memorable performances are filmed in just a few minutes. Other performances are played out over several weeks, and are not necessarily more or less memorable for the time spent getting them on film.
And glorified extras don't have an awful lot of fun, most of the time.