Two things about this, as I like the idea, but feel slightly different about it.
1) I would specifically like to see it as you can offhand dagger (at 50% attack speed reduction on offhand). I feel like dual wielding swords swords or spears could get hard to balance.
2) I would like it to be usable with alternative weapons. as sword / dagger, or spear / dagger. It would give a wonderful little bonus in damage, without breaking balance.
That’s just my two cents though =).