So Jacob is trying to keep the smoke monster on the island but is also bringing people there to test them? That doesn't make sense.
It does if the point is that Jacob is required to have a successor eventually and the only way to find one is to test people who come there under the existing "rules."
In a way, I think Jacob may be a bit like the Endless in the Sandman series. Some members of the Endless (including, ultimately, Dream) died throughout the centuries but were replaced by others who assumed the mantle of that particular aspect. In Dream's case, he was tired of being there but needed to manipulate events in order to get himself killed and create a replacement.
