Typically Mars rovers use wheels and not tracks. I guess Spirit would have better chances getting out of that soft soil should it have tracks. In general, Mars surface structure is not known in advance, so it seems wiser to be prepaired for difficult terrain and so use tracks.
Why do Mars rovers typically use wheels and not tracks?
There are multiple reasons wheels may be prefered over treads. The main ones that apply to the mars rovers I would put as:
Mass is a critical property, especially for space exploration missions. Treads are generally heavier than wheels.
Treaded vehicles only support skid steering and are thus less precise to manouver. They also take more power when turning.
For space exploration simplicity is key. A wheel has less moving parts that can get damaged.
Material properties also have to be taken into account to fit the environmental conditions.
I think the surface of mars was pretty well known by the time Spirit / Op landed.
The rocker-bogie system allows the robot to climb over obstacles up to twice the diameter of the wheels, while avoiding springs completely. Springs could cause the chassis to tilt on uneven terrain. Watch this video: https://www.youtube.com/watch?v=ON1zLBvYKRI, and imagine how much the chassis would tilt if it were suspended by a spring-based system or if the wheels were attached to each other by treads.
The Jet Propulsion Laboratory has a good video on this, actually. http://www-a.jpl.nasa.gov/video/index.php?all_videos&id=932
This may not be the biggest reason, but I would imagine that for space vehicles like this, wheels are easier to stow for launch and then deploy for landing.
