I liked the potential it had set up as mystery of sorts. Even if the answer to the mystery didn't solve any of the other 4 million mysteries in the series, the fact that we knew nothing about the two characters allowed for alot of potential in that arena. However, I no longer considered this a valid excuse for the episode, after its concluded with none of the survivors actually solving the mystery, and the only reason we know what happened was because we physically saw the flashback of the event.
Aside from the complete tangent where Charlie admits he faked the kidnapping of Sun, the only character development was from the two people who left the show with as little impact as they entered.