I tyhink my only problem with the Starship Duty as a prereq was as follows: Starfleet's a big organization...somewhere in there, you're gonna have people who, for some reason, never get into space -- they're doctors serving in TMCs on various planets, paper-pushers who get assigned to Starfleet Command or some such...
So unless the SS Duty just reflects, at the very least, a 3 or 6 month 'cadet cruise', I would think narrator's could exchange it for 'starbase duty' or 'planetside duty'. Just a thought.
An idea for it might be (sorta like the US Navy): you can never go to space and still get your career training, but it might limit your ability for promotion. "Hey, Lieutenant Commander Snuffy, you're up for promotion, but it won't happen. You need a space tour for the rank."
"But Admiral, sir, I've been a starship designer and builder for years. I work at Utopia PLanitia...I don't wanna go to space."
"I understand Snuf...here's your transfer orders. Don't wear a red shirt..."
Just thinking aloud...
"War is an ugly thing but not the ugliest of things; the decayed and degraded state of moral and patriotic feelings which thinks that nothing is worth war is much worse. A man who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself."
John Stuart Mill