</font><font color="blue" class="small">( Two reasons I can think of. One is that a percentage of overspeed is built into the system, with the front tires turning faster than the rears. On pavement this will wear the tread off the front tires.)</font>
That pretty much sums it up Bill. When engaged, the front wheels are helping to pull, but at a different speed than the rears. In soft ground they all tend to spin a little, but not on hard surfaces. Drive train damage, early wear and quicker wear out of the tires is common if doing a lot of work on pavement.
espn, if you have a front end loader (FEL) on your tractor, just lift the front slightly with the loader and carefully and slowly press the accelerator to see if the wheels are turning. Then move the lever mentioned earlier to the other position and confirm that the stop turning. Mark it so you always remember which position. John