Quite the contrary, I believe NASA is very serious about meeting their goals. If the goal was to fly for 30 days, then they'd have a huge challenge in over-engineering the mission to ensure 30 days. This way, they can get the initial proof done (hooray!) and then run the robot to failure (which itself will be interesting.) It might fail in 30 days. It might not.

Another benefit of this approach is, as with Spirit & Odyssey, they can use off-the-shelf components if they meet the initial goal.