IF you assume that you can throw multiple weapons with a single throwing action, I would treat it as firing two guns, or attacking with two melee weapons. Split your die pool by the number of weapons you're throwing and resolve each as a separate attack.
IF you assume that you can throw only one weapon per throw action, then the advantage of readying multiple throwing weapons is that you don't have to take another ready action until you run out.
For example:
AGI 6, 3 IPs
1st IP:
FREE: Taunt your soon-to-be pincushion target.
SIMPLE: Ready 3 throwing knives.
SIMPLE: Throw 1 throwing knife.
2nd IP:
FREE: Laugh at your enemy's pain as he grasps at your throwing knife.
SIMPLE: Throw 2nd throwing knife.
SIMPLE: Throw 3rd throwing knife.
3rd IP:
FREE: Call out your next target.
SIMPLE: Ready 3 throwing knives.
SIMPLE: Throw 1 throwing knife.
I think it's pretty clear how advantageous it is, compared with having to draw, and throw, each initiative pass. You're rate of fire at AGI 6 is 1.5 throwing knives per IP vice 1.