Very very inconvenient. Writing on air is completely different and considerably more tricky that writing on paper. Physically more demanding too. It's one of those things that sound nice in theory, but not really practical.
But we can imagine a kind of ASL shorthand, like the original Palm Pilot's 'Graffiti'. Or, maybe if you buy two or three, you can set them up to combine perspectives, giving them more insight into the full 3d space.
That could be useful to deaf people and maybe could also serve as a new form of shorthand for people who are writing long texts on devices without keyboards.
You can even network more than one Leap device, to create even larger interaction areas.
I imagine two units orthogonal to each other might be enough to capture ASL.
-Chris
I don't see traditional writing being a realistic application, though perhaps cursive signatures could work.