It was one of the first (and best) mods I did.
If you need it (as in: your driving style suits it), you probably already know it.
But indeed, if you're not the full throttle guy you dont need it.
And you're not buying it for the 95% of the situations, but for the 5% of the situations that the car struggles with grip, so the difficult grip situations.
DTC is no option as it's way to intrusive in the z4. But an LSD works very good in combitation with, because the point at witch DTC is activated is much later, as an LSD actually gives more grip and traction.
But mind you, its a mod that gives 0% streetcred. If you want something flashy, get something else.
You do this because it complements your driving style. So if you're a slow driver its a waste of money. And if you're a fast driver it's one of the best mods.