From Wayne Hale’s blog-
On April 29, 2002, the newly confirmed NASA Administrator Sean O’Keefe planted a time bomb in the International Space Station program. Coming to NASA from the US Office of Management and Budget, where he was Deputy Director, Mr. O’Keefe had a reputation as a bean counter and penny pincher. Mr. O’Keefe publicly joked that he was not smart enough to be NASA Administrator. But he knew the ISS program needed political capital in the US political arena. Providing a big, flashy cut in the ISS program would cement O’Keefe’s position as NASA Administrator and aid in the annual budget fights in the US Congress. One part of the ISS program caught his attention: the plan to develop a US “lifeboat” for the ISS. Since the Russian Soyuz could fulfill that job – and the Russians were providing that service as part of their initial contribution to the international partnership – the Crew Rescue Vehicle (CRV or X-38) became an easy cut. So on April 29, the total dependence on the Soyuz for the life of the ISS program was established by NASA fiat, with virtually no consultation with the other ISS partners…
…We could have really used that CRV. It might even have become the basis of a mini-shuttle crew transport vehicle. But no; it was eliminated for the most transient and banal of reasons. The old adage against being “penny wise and pound foolish” has struck the human space effort once again.
Or more to the point; every good space designer knows that a system with a critical single point failure is not a good system. Reliability is key, but even then, having redundancy is the standard practice for a truly resilient system.