As I see it
Disarming:
- removes chances of Parrying
- Most of us don't have Cortosis to stand up to Lightsaber-hits (most Force Powers let you benefit from normal Soak).
- Might lower Defense?
Removing Defense:
- Removes 4 Setback dice for one round
- Most of us can't ignore Setback dice of that magnitude
There's also the possibility that he can just Force Move it back to his hand, or have abilities to avoid Disarm attempts though.