The stock differential is a trac-loc, meaning both wheels are spinning at once. If you upgrade to a 31 spline differential you can upgrade your axles making them stronger. This allows for more hardcore burn outs and slicks at the track. Ford trac-loc's are dirt cheap and hold up just as good as the more expensive Auburn/Detroit units.
Actually the stock differential is _not_ a traction lock (we are really talking about the carrier here anyway). Ford uses a traditional clutch-type carrier. The two rear axles are locked together thru a set of disks/pads that can slip. They use a spring to press the pads together, which controls the "locking" between the two wheels. However, if you put the car in neutral, and raise one rear wheel, you might be surprised how easy it is to break that locking friction and spin that one wheel.
BTW the ford limited slip is _not_ as strong as auburn, et al. The ford carrier is dirt cheap, and as usual, you get what you pay for. After a year (or less) of hard use, it will act just like an open rear end as the clutches will be slipping so bad they are ineffective...
Better carriers (auburn pro, etc) have much stronger springs, and a much better clutch mechanism (cone), which provides a much better lock between the two axles, still allowing them to slip when going around corners.
traction-lockers mechanically lock the rear axles under heavy torque load from the pinion gear, but they can "slip" when going around corners (but not accelerating very hard) to avoid excessive tire wear. newer flavors of these are very quiet (older versions made a lot of noise going around corners).
The ultimate is a pure spool where the carrier becomes a solid piece connecting the two axles to the ring gear, no slipping possible. Not good for street. Great for track.
The stock clutch system (talking rear end here) is very weak. The bigger the rear tires, the more likely you are to get a 1-wheel burnout which is no good for launching.
If you don't run at the strip, there's no advantage to swapping this out at all... And doing so will increase rear tire wear a bit as it is harder to make the clutch slip, resulting in scrubbing off rubber as you go around corners.