Great question! JPL engineer Ravi Prakash answered a similar question on the JPL blog: "Because Mars Science Laboratory is so much larger than anything we’ve sent to Mars, none of our previous landing designs would work, so we had to be innovative. While innovation can have high risk, it can also have high rewards. Without innovative ideas, we would never have landed a man on the Moon or even gone to space in the first place. The moment we are afraid to try to put innovative ideas into practice is the moment our society ceases to progress. While the Sky Crane landing system sounds and looks cool, it was chosen because it is a very elegant design. One of the benefits it has over previous landing systems is that we’re not bouncing around on the Martian surface, so we don’t have to protect the rover from hard impacts. Also, once we touch down on the surface, we don’t have to worry about retracting airbags or egress from the landing petals, so we are pretty much ready to start driving. The list of advantages goes on and on... An interesting side note is that many people thought an airbag landing system was a crazy idea, but that landing system ended up working great for the Mars Pathfinder and Mars Exploration Rover missions!" You can read more from Ravi's blog post at http://blogs.jpl.nasa.gov/?p=11 In terms of your question about a rover in a ball, you might be interested to read more about JPL's Tumbleweed concept rover, which was designed to traverse a planetary surface with a rolling and/or bouncing motion driven by the wind: http://www-robotics.jpl.nasa.gov/tasks/showTask.cfm?FuseAction=ShowTask&TaskID=91&tdaID=888885