Had she stayed, we would have had no "Yesterday's Enterprise" the same way, or Sela. We'd have probably still had an altered "YE" and no Sela is a good thing by me. Worf wouldn't have been as overexposed, and might have gone to DS9 from the start, two more good things.
Eh, I don't think Sela herself is a problem... I think the fact that they went out of the way to create this character, use her only a few times, build her up, but still we wonder "Why bother with the Tasha Yar's Daughter thing? What does this add to the story?"
Ultimately she was useless. Besides, we had a much better Recurring Romulan character. Tomalok.
Tomalok only had a few short appearances but was actually a lot more memorable and was a much better thought out character.
I think Yar dying in the line of Duty was the way to go... What actually impressed me about the episode was, they didn't have a long "Maybe we can save her" sequence where everyone tears up. She's dead, in sickbay, nothing they can do, story moves forward. Gets the memorial at the end,and the episode's done. To have that happen to a major character is actually quite ballsy.
Much better than Jadzia's fate in Deep Space Nine.