This ability actually sounds like a great benefit in doubles/triples.
There's the "Beat Up + Justified" combo, which I don't know if it works, but it can instantly give +6 to the Justified Pokemon. All of the Justified users bar Gallade and Arcanine resist Beat Up too.
Another benefit is from Fling. When the stat berries are released (or if they are already released), one can fling something like a Salac Berry at a Justified Pokemon. Because Fling is a Dark Type move, Justified activates and an additional boost is gained. You could also have the Justified Pokemon use a boosting move the same turn. For example, flinging a Salac Berry at a Cobalion who is using Swords Dance ends with +3 Attack, +1 Speed.