OK. I see what you are getting at... And you are indeed correct that the most common roles aboard ship are as you indicated. In many cases the mission specialist and redshirt as NPC's are equally important, because the specialist knowledge required in one plot may relegate the character to insignificance in another.
One option is to allow 'guest stars' for non-regular players, or troop play which allows your players to take a secondary character to use when the plot is less suitavble for their main one (the GM nod that the secondary characer is in play might give them a clue as to whats ahead...)
But there are other ways for a character to shine. Starfleet Officers are supposed to be renaicence men, masters of multiple disciplines. So that Engineer might also be a top notch pilot who authored a thesis on temporal relativity. The Civilian El Aurian councellor might actually be a former Starfleet science officer, The Ops Officer might be the ships second officer...
Soetimes al it takes is to think a little bit outside the box.
DanG/Darth Gurden
The Voice of Reason and Sith Lord
“Putting the FUNK! back into Dysfunctional!”
Coming soon. The USS Ganymede NCC-80107
"Ad astrae per scientia" (To the stars through knowledge)