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Woljags, It was something along those lines I also thought about. But maybe not as practible as it sounds. It is sort of saying, we have them anyway. They are paid for, why not use them a bit longer. After all, you don't ditch the car from last year, still in a mint condition, because you plan to buy a new one in five years time.
Maybe the additional cost of putting a better suited inter-planetary vehicle together is insignificant compared to the other things you will need. HOWEVER, NASA is not exactly known for cost optimisation.
Ivan