Customer service is telling me that I need to have at least one cable box on my account if I wanted to add a Cable Card to the plan.
It might depend on the offer. I can go to the TWC order site and configure a cablecard-only plan but the promotion they offered was cheaper but included DVR service. I don't expect to use any two-way services but given the DVR I can. I also got a two tuner TiVo so I can attach my antenna so the extra tuners might come in handy.