Hey Eric,
My experience with a straight track(Reynolds 21) was that when you sheeted the jib hard to flatten the sail for upwind work, the clew would move towards centerline because the distance between the jib car and the jib's tack is shorter the closer the car gets to the mast. This would close off the slot too much. This is mostly a problem in ligher air, when it's blowing harder there can be enough pressure to force the clew outboard.
With the curved track, you can sheet hard or soft without the tension influencing the clew position relative to centerline, as the distance between jib tack and jib car is always the same. This allows jib shaping with sheet tension without changing the clew location.
That make sense?

Dave