It wasn't just Ben seeing his mother that interested Richard, it was the fact that she died off the island that seemed to interest Richard the most. In the commentary for the episode, "Man Behind the Curtain," Lindelof says to take particular note of that fact.
Interesting...
have there been any other "ghosts" we've seen of people who died off island? I guess Christian died off island, but his body was brought to the island. Same with Locke. He died off island, but his body was brought back. Yemi died off island, but his body ended up there. Jacob died on island, but then his body disappeared.
Ben's mom died off island, but her body was never brought to the island.
All of the "ghosts" Hurley see's have died on the island (except Dave, who was never alive to begin with).
Miles can talk to dead people, both on and off the island.
Locke, Shannon & Sayid all saw "ghost" Walt, but he's still alive.
GhostChristian has appeared both on and off the island (and back in time).
The Ghost's Hurley sees have been while he was off the island all off island (right?), but all of people who died on the island (Charlie, Eko, Ana Lucia) except for Ghost(?)Jacob who he saw on island (and Dave, who might have just been in Hurley's mind).
Is there some pattern here?
If we assume that Christian, Yemi and NotLocke are all created by the MIB, then it seems he can only "create/use" the bodies of people who died off island, but who's bodies were brought to the island.
If all the ghosts Hurley sees are created by Jacob, it appears that Jacob can only create/use ghosts of people who died on the island.
Not sure if that is correct, or what it might mean.