In general it's better to have multiple working designs. First, in case something happens with one, another can keep the traffic; that's what happened e.g. with ISS manned flights, when after the Columbia destruction flights moved to Soyuzes. Second, market - if the service is bought from a commercial company, competition helps keeping prices in check. Cost plus scheme, used by the NASA for a long time, does a rather poor job, evidenced by the USA leaving international launch market before the SpaceX coming.