If you try to build a power that is at-will and start talking about scenarios where with a single attack (or even a series of actions) you can reduce the effectiveness of an opponent (PC) by 50% (or more) for the rest of the encounter then everyone will be using this power above ANY others.
I think you overestimate the power of disarms.
I ran the first battle from KotS, using disarms at-will. Effective? Ineffective? You be the judge. Next time I'll pick something at a higher level where there's an elite monster that relies on a weapon and see how that works out.
[sblock]Init:
M 23
S 5
K 16
A 8
B 10
C 18
D 18
E 15
Round 1:
M1 moves up, charges the cleric. 16 v AC 16; 4 damage.
M2 moves up, charges the cleric. 11 v AC 16.
M3 draws a javelin, moves up, throws at the cleric. 17 v AC 16; 4 damage.
M4 draws a javelin, moves up, throws at the cleric. 10 v AC 16.
M5 draws a javelin, moves up, throws at the cleric. 6 v AC 16.
Dwyver the elf ranger moves next to the bushes, twin strikes both minions next to the cleric. 13 v AC 16, 25 v AC 16. M2 is killed.
Castor the dwarf cleric hits M1 with priest's shield. 15 v AC 16. He shifts forward a square.
K1 (Dragonshield) moves up and charges Albert the human fighter. Crit; 9 damage, and he's marked.
K2 moves up and charges Castor the cleric. 24 v AC 16. 9 damage, and he's marked.
Edgar the tiefling fey-pact warlock curses K1 and fires an eldtrich blast at him. 19 v R 13; 12 damage.
Betty the human wizard drops a cloud of daggers on M1. 7 v R 13.
Albert the human fighter attempts to disarm K1. 24 v AC 18+2 (he has a shield, so AC is fine). K1 drops his short sword in his space.
S shifts, loads a firepot, and fires at Castor. 18 v AC 16; 5 damage and 2 ongoing fire.
M2 dies from the cloud of daggers.
M3 draws his spear, moves in adjacent to Castor, readies for when he has flanking.
M4 draws his spear, moves in adjacent to Castor, readies for when M5 gets adjacent to Castor.
M5 draws his spear, moves in adjacent to Castor.
M3 attacks: 22 v AC 16, 4 damage.
M4 attacks: 13 v AC 16.
M5 attacks: 17 v AC 16. 4 damage. Castor falls at -3 hp.
Dwyver the elf ranger moves to get a clear shot at M3 and M5, then twin strikes them. 22 and 13 v AC 16. M3 dies.
Castor makes a save against the fire. 6, fails. Makes a death save. 3, fails.
K1 picks up his short sword and shifts forward a square, adjacent to Albert and Betty. Albert makes a disarm attack as an immediate reaction: crit. K1 drops his sword in his square. K1 readies an action to kick Albert when he's flanked.
K2 moves forward, shifts to flank Albert.
K1 attacks Albert: 16 v AC 19.
K2 attacks Albert: 20 v AC 19. 5 damage.
Edgar moves over a step, curses K2, eldtrich blasts K1. 14 v R 13; 21 damage.
Betty delays until Albert goes.
Albert disarms K2. 24 v AC 18+2. Shifts back a square.
Betty thunderwaves K1, K2, and M5. 20 v F 14, 21 v F 14, natural 1. K1 & K2 take 11 damage. K1 is killed, K2 is pushed back 2. As a minor action she uses Mage Hand to throw the dragonshield's short sword 5 squares away, behind the PCs. Betty spends an AP to thunderwave K2 and M5 again. crit, 15 v F 11. M5 dies, K2 takes 11 damage.
S throws a firepot at Albert. 15 v AC 19.
M5 moves up, grabs K1's sword, and throws it to K2.
Dwyver twin strikes K2. 20, 23 v AC 18. 14 damage. K2 is killed.
DM calls the fight over.[/sblock]