Correct, part time 4WD is that, for part time use.  If the traction is
high enough to not allow wheel slippage, you don't need 4WD anyway,
shift into 2WD.  The reason is that the transfer case does not allow
slip between the front and rear axles (i.e. there is no center
differential).  To see why this is important, take any vehicle, drive
through a puddle in a parking lot, then make a sharp u-turn.  Hop out
and look at the tire tracks.  Note how the front tires swing in a wider
arc overall than the rear tires.  Wider are = longer distance travelled
= more wheel (and thus axle) revolutions.  If the front axle makes more
revs. than the rear, something has got to give.  Normally, off-road, you
have lower traction and thus the front or rear tires can slip to make up
the difference.  On pavement, this is much harder to do, so a lot of
stress builds up in the drivetrain.  This is what you want to avoid.