Dueling in TSL is pretty much a waste of time and feats.
As one's character level progresses the apparent bonuses incurred by dueling become grossly less significant. This is party due to the fact that all Jedi Classes have equal BaB progression. Hence the +3 attack bonus from Master Dueling is rendered insignificant at higher levels, as you'll effectivley be able to hit anything anyway.
Also, both regualar Jedi classes and Prestige Classes gain hefty class bonuses to defence. Therefore, using the aforementioned reasoning, the +3 defence bonus from Master Dueling somewhat also pales in comparison.
As Hades said, with the exception of the earliest of early levels, once you invest some feats into Two-Weapon Fighting, it will always be better than Dueling - twice as good in fact.
* observes Number Man & Epiphany employ Master Dueling, leaves room quietly :ph34r: *