Its weapon sets grant effects of other skill tree abilities to the effects of that weapon's tree abilities. You must have purchased the ability or it is useless.
Might even increase the cost of the ability/abilities it modifies.
For instance, Defensive Stance could gain Blade Cloak, granting Minor Evasion (instead of Major) and dealing area damage around you, also only lasts 6 seconds like Stance.
Puncture could gain the range and interrupt power of Hidden Blade.
Morphs and ability levels should alter/enhance the power's buffs.
Quick Cloak: Defensive Stance gains Major or Minor Expedition.
Deadly Cloak: Increased AoE damage.
Shrouded Daggers: Puncture can bounce when cast from 7+ meters away, hitting more targets.
Flying Blade: When you cast puncture from 7+ meters away, you can recast on the same target to leap to them.
Other abilities, other possibilities, too many possible combinations. Now I wonder if that's why that hasn't been done yet.
With these sets, it would encourage people to level ALL their weapon trees, not just the one they use all the time.
I use shield with a side of Ice Staff, and don't use the others. The above would have me use the others to improve my main stuff.
I wonder what combinations the devs come up with.