Something was made very clear in "Surviviors:"
Haven has no intention of exploring the new persona that has been thrust upon Audrey/Lucy/Sarah during her last trip into the barn, so keeping Lexie around is a complete waste of time. It's also incredibly boring. Emily Rose and Colin Ferguson are too talented to throw the same phrases at each other over a bar the whole time they're on screen.
When Lexie picked up the gun, put the pieces together, aimed and shot, I literally groaned and said, "thank God!" Yet we still ended the hour with a useless cliffhanger of William telling Lexie once she knows her true identity there is no going back.