Roxanne certainly was one very beautiful lady, beautiful enough to take Barnabas' mind off Josette. Little wonder he did not [spoiler]want to do away with her, even if she was the life force for the evil Angelique of PT; and also balked at destroying her vampire counterpart in regular time, having her coffin chained up instead (even from there she was able to summon Maggie and tell her to break the chains). It was this series of events that prompted Sebastian Shaw to drive her to Wyndcliffe. Had she not been under Roxanne's power, there might not have been reason for him to take her to Wyndcliffe, and she might have perished in the attack on Collinwood by Gerard's zombie pirates.[/spoiler]
I wonder if Barnabas, when he was chained in his coffin in 1795, didn't have someone in his power he could summon to break the chains of his coffin, drive a stake through his heart and end the misery of the vampire curse.