Going downwind last weekend in light air, I stumbled across another reason to keep the boards fully down in light air (at least on a 2005 Nacra F18).

With the boards raised ~30 cm or so, I had to keep the rudders 20-30 degrees to windward to go straight. Put the boards down, and it's back to neutral helm. With more wind (and more speed through the water), less daggerboard area is needed to balance out the sail plan.