"Most people don't realize that these spacecraft are always launched on a rocket that is just on the edge of its performance capabilities. If we had a big cheap rocket that was not pressed to its limit, the launch date opportunities would be more plentiful, and launch windows would be bigger. It's also a mater of how much money you have available to spend on a rocket. "

Hmm, I don't know what you mean by "Delta 2 heavy"- how does it compare to Zenit which launched by same company's Sea Launch?
[payload & costs]
The Delta 2 class is generally fairly reliable- unlike the Delta 3.
Most people don't realize that the expected reliablity of a launcher is a determining factor of the value of a rocket. NASA could choosing this "Delta 2 heavy" because it's a proven rocket or the opposite, they could decide that there is "some value" in helping to prove some rocket (and getting lower launch price).
Once you understand that a rocket value is related to it's track record, then you can begin to understand why I think the govt should buy water test payloads.