I have heard of other jobs using the dagger to trigger the haste and switching to their best weapon, often a different type of weapon.
The haste effect from what I read, lasts 3 mins.
It's not a bad dagger at all if you haven't been working on a trial of the magian weapon or a relic/empyrean/mythic/WoE weapon.
If you have better knives, I'd treat it like you treat Treasure Hunter knife. Put it on your sub slot until you trigger the effect then switch it for something better.