It's only as overpowered as it is because of how overpowered twin strike is. But combined with any other "this at will attack is better than other at will attack" powers, half-elves can now really bring on the cheese -- get it as a basic melee (if it's a melee attack), recast it to use the half-elf's best stat, and now get a free attack with it whenever they spend an action point (as a pp) and combine it with the rather absurd things you can do in beast form (eg, claw gloves, dire totem [with close burst attacks], Enraged Boar/Ferocious Tiger form). That we're using Twin Strike as exemplar is pretty clearly explained by TS being, well, absurd; I'm not sure what the next few good at-wills are after TS. Magic Weapon? Righeous Brand? Piercing Strike? Devastating Strike? Dragon's Tail? Five storms? Sword Burst? Dual Strike (probably not unless you're a chainfighter or a whip+flail fighter)?