Continuous track have some significant disadvantages for the purpose of a remote probe.They add extra weight which makes the launch and landing more expensive.Mostly they are prone to getting jammed up with junk. This isn't a problem for a multi tonne bulldozer with a multi kilowatt power plant that can pretty much crush and brute force away all but the worst blockages. For a light, low energy machine like spirit and opportunity this brute force approach to clearing the tracks is not an option. And finally, if the track breaks the probe is pretty much stuck,A multi wheel system is a more efficient way to approach the problem. With multiple, independently driven wheels, you have a nice redundancy factor. Wheels are less likely to jam up with random junk.A better way to approach the current problem for future probes is to add some mechanism where the wheels can be locked up and the whole unit can be "walked" out of the sand. Although this does add more mechanical bits to fails it does allow for more complex terrain traversal.