Well, there are a lot more burst/blast type powers with implements powers than with weapon powers, which likely makes the use of nightbringers debuffing much more efficient - rather than negating it from one monster at time (and maybe not focusing fire), you're doing it to several at a go.
That said, it really only matters if either the players have a lot of necrotic damage (unlikely) or if they can get monsters with necrotic damage to damage each other. The biggest problem would be with enemies with necrotic auras (and to a lesser extent, burst and blasts) - suddenly they damage each other if they get close to each other.
Personally, as noted, I would limit it to weapon attacks - with implement attacks, the implement doesn't normally come into physical contact with the target. Taking the language out of the D&D context, its like saying "the felon struck the cashier with his gun then fled" - its implicit that the felon used the gun as a bludgeon and didn't shoot the cashier with it.
Balance wise, it likely isn't too big of deal either way, though the ability to apply the debuff to several targets at once will take certain encounters apart (namely, those featuring baddies with necrotic auras).
Ninja'd by KarinsDad