The consensus seems to be that having an unsharpened sgian dubh is useless.

I agree, that as a tool, a knife that is dull is less than useful and can be downright dangerous.
As to my earlier post about the legalities of carrying a sgian dubh, as Katia pointed out, intent can be what makes it legal or illegal to carry. So, in Florida; if you carry a knife with the intent to use it as a weapon, then it requires a concealed weapons permit, if you carry a knife to open packages, slice bread, cut vegetables, graft trees or such, (use the knife as a tool), then it does not require a concealed weapons permit. So it really boils down to what the investigating officer, (if you are stopped for some reason or another), thinks you are intending to use the knife as: a tool or a weapon.

That makes things as clear as fermenting whisky mash....