A Less Efficient Conventional Propulsion System?

Can someone tell me what this means? From the Government Accountability Office report (PDF) comparing conventional and nuclear propulsion for the CG(X).

We recommend that the Secretary of Defense require that the Navy (1) before finalizing Phase 2 of the Maritime Air and Missile Defense of Joint Forces Analysis of Alternatives, include present value analysis, alternative fuel scenarios, and analysis on the effect that a less efficient conventional propulsion system has on the cost estimates and (2) include present value analysis and alternative fuel scenarios in any future analyses of the trade-off between conventional and nuclear propulsion.

What am I missing here? What is the purpose of comparing a less efficient conventional propulsion system? The report described the ships compared as:

In the CG(X) Analysis of Alternatives, the Navy identified six ship design concepts. These concepts include developing new designs as well as making modifications to previous hulls. For example, two concepts are based upon making modifications to the DDG 1000 Zumwalt-class destroyer and another concept is based upon making modifications to the DDG 51 Arleigh Burke-class destroyer. The ship design concepts vary in both capability, including the sensitivity of the radar and number of missile cells, and propulsion system. The variability is based on whether the concept uses a previous hull or is a new design. The Navy analyzed two new cruiser design concepts, one with a conventional propulsion system and one with a nuclear propulsion system. Both included the most sensitive radar and highest number of missile cells of all the concepts.

Are we led to believe the new conventional cruiser supposedly used a highly efficient propulsion system, and instead it should use a 'less efficient' propulsion system instead? If two of the concepts are based on the DDG-1000, and one is based on the DDG-51, why would the report recommend using a less efficient propulsion system than what already exists on these existing platforms?

This report reads like Greek to me (because it intentionally gives few details), so I ask the audience for assistance in translation.