Should he stay or Should he go ?

Richard Malkin told Claire that she should raise him herself.

Then gave her a ticket to go and give him away.

Claire thought that Malkin knew that they would crash- and survive(?).

So somehow, we were lead to believe that it was a good thing for Aaron to be on the island with Claire. I personally thought that Aaron was maybe special like Locke, and would replace Jacob someday.

But now, Claire's ghost(it's either the island/jacob or Kate's guilty-conscience) haunts Kate and tells Kate that she should return to the island, without Aaron.

On the other hand, Ben, who usually knows about everything, advises Jack on how they should all go back, each and every one of them, including Aaron.

So all of this, combined with Faraday's warning which reveals that their team doesn't know how to get back to the island once it disappears and Ben might be the only one who does, made me think that perhaps the island knows that leaving Aaron behind might help them to have a connection and to be able to return to the real world or to be found- should the island get lost again.

Any thoughts on why the island doesn't want Aaron to come back ?

I think we should be careful about labelling Claire as a ghost or anything else at the moment. Her appearance for Kate was, we shouldn't forget, only in the space of a dream.

We don't know what's become of the actual Claire.

It may well be that Claire became convinced that Aaron should not remain on the Island because the likes of Christian and Jacob (or whatever) convinced her it was the right thing to do.

And maybe there are opposing factions - like Alpert or Abaddon - that Malkin was also aligned with that believed Aaron should be on the Island.

good questions/points and equally good response by AC. I would add that besides Claire only appearing to Kate in a dream, I think it might be faulty to rely on Ben to solely promote the island's best interest. Jack noted that Locke had tried to get him to come back, as did Hurley. Ben may have his own motivation for Aaron returning to the island independent of the island's welfare. Much like when he killed Keamy--seemingly for revenge, not out of duty to the island.