Ryujin
Legend
He's MCed, not Hybrid.
The PHB says "when you damage", but I think it's the default after they did some revisions. I could be wrong. The hybrid rules just codify it.
If it isn't the case, though, what is the benefit of a Pact Weapon?
If it's MC, then you should be able to apply Curse at will. "If you hit a Cursed enemy with an attack, you deal extra damage." The current Compendium listing doesn't specify that it be a Warlock attack, though the Hybrid rules do. Probably in order to avoid unwanted and unbalancing interaction.
The value of a Pact Weapon is in its ability to be used as an implement and in the additional riders it has. A Pact Blade Dagger is a damned fine minion popper.