"While the failure of a launcher carrying only consumables might be regrettable, it’s not because of the payload value. If many tons of commodities were to be delivered annually and launched in one-ton shots, the loss of one shot would be written off and a replacement launch performed in short order. A number of replacement launches would in fact be expected and included in operating plans. Furthermore, if the consumables were launched in advance of need and maintained on a depot until required, the orbital user would never experience a delay in deliveries or interruption of service even if a commodity-carrying launcher suffered a failure!
...
Addressing the American Astronautical Society on November 15, 2005, NASA Administrator Mike Griffin said “…specifically we should desire to place fuel in orbit by the cheapest means possible, in whatever manner this can be accomplished, whether of high reliability or not… if there were a fuel depot available on orbit, one capable of being replenished at any time, the Earth departure stage could after refueling carry significantly more payload… maximizing the utility of the inherently expensive SDHLV [Shuttle-Derived Heavy-Lift Vehicle] for carrying high-value cargo.” "
http://www.thespacereview.com/article/544/1
[linked from hobbySpace]