I am surprised that I have heard no serious suggestion to build at least one more of the spacecraft used in the next mission to study an asteroid or comet(eg the DAWN project), and put the extra spacecraft in storage.

That way, if there is ever a serious asteroid threat, you can launch it on the first heavy-lift launcher available. An Ariane 5 could give a heck of allot of Delta V to a spacecraft intended for a much smaller launcher. You have a proven spacecraft available to give you a very quick look at anything that comes up.

It would also be available for any extraordinary scientific oportunity, or to act as a low-cost communications relay in the event of a Galileo-type failure in a more expensive spacecraft.

You may have to design the craft from the start with this potential use in mind, so it might add a bit to development costs. But the cost for an extra copy of the spacecraft is really minimal compared to the first one, or so I have heard.

You could go even further and build 3-4 spacecraft for the next mission. One goes into storage, the others are launched and return far more science per dollar because of the low additional cost of building the extra spacecraft. This is why NASA is using 2 MARS rovers, I believe...