I'd say the point where it matters is the point where you need cornering traction. For a commute on a dirt road I'm not feeling any lack but if I were to take it on the single track parallel to the dirt road I'd rather have the knobs.

Also remember that different knobs really do different things. There's a spectrum of wet to dry and packed to loose and you need to read between the lines on the marketing bumf to pick the tire for the dirt.