Well, we did see an entire show with two characters working together after a break up... though one that happened in the past (Riker & Troi).While Trek has done the start of relationship well, we've never seen one between two main characters break up. It would be interesting to see how they'd handle two characters that have to work together endure a relationship breakdown.
And isn't what you mention kind of what was going on with Trip and T'Pol, though they didn't as much break up as had a never defined on off relationship?
It's the Love Interest of the Week syndrome. Trek writers usually seemed to think that any romance with a non-main character should be over by the end of the episode and the love interest had to either 1) die, 2) leave/be left behind, or 3) turn out to be a psycho killer or something.Danara Pel is one "alien babe of the week" I would have welcomed into the Voyager family.
The EMH should have beamed HER aboard, when he beamed the serum over in "Resolutions". Surely she wasn't long for this world after the Vidians found out what she had done?
