Must agree with Kevin. The Captain, and George Kirk making that sacrifice, steals too much thunder from Kirk and the dramatic sacrifice he makes. The story calls for Kirk's ultimate sacrifice. Kirk's and Kirk's alone. Yes, Robau bought time for his crew, and that was a noble sacrifice. But it wasn't the point of the scene. The point of the scene was to show George Kirk's ultimate sacrifice, and little Jimmy T's. start on his reputation for "dodging death's hand".
The argument you make for Robau is flawed. Robau was a character that wasn't really meant to be heartfeltedly (yes, I made up a word) cared about...George Kirk is the focus. He is the one the audience is meant to be drawn to, to achieve that emotional connection, the empathy and sympathy.
So, again, (and Star Trek be damned because it falls into the same category as any other work of fiction, regardless of "heady ideals", or any other fluff that one might associate with it. It is a work of fiction, and subject to the same needs for dramatic tension and purpose as any other), logic and physics yield to dramatic need. The purpose is to move the audience, not bore them with the hows and whys of one particular decision. (For that, we have Star Trek TNG's infamous "Captain's consultation sessions" before every decision Picard makes.)
Role Playing Games often fall into this style of narrative. The GameMaster (DungeonMaster) is the storyteller of the game. The players are both audience and actor. The GameMaster, as director of the narrative, must direct not only the story, but the attention of the audience to the narrative he intends to tell. The characters only have so much free will in an RPG. The GM must guide and direct its characters and audience back to the main narrative...even though the director can occasionally indulge the players' whims. And yes, GM's are often encouraged by the rule books to "fudge the rules" every now and then if it is necessary to keep the pace going, or to get the players back on the right track.
It is the same with movies, only less free. If the movies were to follow the free will of its audience, we would not have a conclusion at all....it would be chaos. No. The movies are told with a specific intent...to take the audience where the story is supposed to go. Sometimes that means fudging logic and physics...the rules...to pick up the pace or get the audience to where the movie is supposed to go.