Ben made numerous trips off the island and returned without a problem.
Ethan and Alpert recruited Juliet and returned with her without issue.
So did Mr. Friendly from NYC with Michael.
So the island was never really lost.
But you remember, the island time-space skipped after Ben turned the frozen donkey wheel. That is why the island was lost; it moved. Except that Ben had turned the FDW wheel before, and landed in North Africa and stayed at the same hotel. And in those earlier time jumps, he returned to the island without the help of the Lamp Post station or Eloise.
So there is a major inconsistency in the story lines.
It was a plot convenience to add tension by making the island "vanish," but it runs contrary to pre-existing facts about the island and how it worked.
But then, the answer must lie with Jacob. He was the "only" person who could allow individuals onto his island. That would mean that Jacob would micromanage all the comings and goings of the Dharma Group, the Others, and the 815 survivors. But Jacob really was not shown as a "hands on" guy. In fact, he delegated any supervision of the island inhabitants to Alpert. Jacob may have allowed people onto the island, but gave them the free will to make their own choices to see if corruption would lead to their demise.
So if the island was never truly lost or hidden, what was the point of the O6 return?
Filler.
We were shown more of the O6 mainland stories than the three years in 1970s Dharmaville.
It is probably because the writers had run out of island stories or were bored with re-running the Horace vs. Others theme.
It also gets to an odd clue that the smoke monster as Christian told Locke when he turned the FDW. Locke had to die in order to return his friends to the island. Locke could not get his friends to voluntarily return. Eloise could not find the island location. But when Locke was killed, Eloise found the island and Ben with Jack reunited the O6 to return on Ajira 316. If there was a cosmic puzzle box to unlock the location, Locke's death was the key to open it. It is a sinister conclusion that a man must die in order for the island to be re-located. What a demanding toll for those travelers, who never realized what, if anything or anybody, was left on the island. It is these questions that show that it was not the island that was lost, but the continuity and forethought in the scripts.
