To explain this, it's first necessary to understand one of the really wonky aspects of gearing up in TOR: the variable value of shield and absorb. Defense has a relatively static value: each percent of Defense chance you get provides 1% damage mitigation against M/R attacks. Shield and Absorb aren't the same. Since Shield chance represents the chance that you have to reduce the incoming damage of an attack by the amount of your Shield Absorb, the performance of a relative improvement is a variable quantity: 1% additional shield chance whenever you've got an absorb of 30% represents only a .3% reduction in incoming damage whereas, with 70% absorb, it would equate to a .7% reduction in incoming damage.
As such, while Defense remains largely static in value, Shield and Absorb vary based upon their relevant listed percentages. Since the listed percentages are a function of your static talent bonuses combined with your gearing but DR only affects gearing, you need a lot of bonuses from talents to shield and absorb to make stacking Shield and Absorb valuable: without the native bonuses to Shield and Absorb, it's not really possible to get to the point where stacking Shield and Absorb to appreciable extents (re: more than just the token quantity that you see listed in the itemization budgets here) is truly effective thanks to diminishing returns. As such, since the Defense tree has a single, highly anemic Shield talent, no absorb talents, but a friggin' *ton* of Defense from talents (3% from Single Saber Mastery, 6% from Guard Stance, 3% from Blade Barricade), Shield and Absorb are very low value which makes Defense very high value. For comparison, Shadows get 20% additional Shield chance and 4% additional Absorb (along with 6% Defense chance) and Vanguards get 8% additional Shield chance and 12% additional Absorb (with only 2% additional Defense).
Thanks. That's very helpful.