re: I think the MER rovers should be mass-produced
I've patiently waited for this mission for years and have seen many flybys including the outter solar system Voyagers and Pioneers. I understand the cost of an orbital mission to Pluto the political difficulties of the same and have patience.
Making more MERs isn't going to happen and would have been a waste of money unless they carried a different set of instruments to Mars. It is entirely possible no more mission will be sent to Mars after the Phoenix Lander unless we protest the cancellation of the MSL when it happens.