Tim,

They don't.

Curved foils are used in trimarans instead of straight ones for practical reasons only. They enable docking with the boards in any position and can be "hoisted", unlike the straight ones.

Curved wings in planes/gliders are similar to dihedral, they generate slip-roll coupling. As far as I know, slip-roll coupling presents designers with a dilemma: it increases roll-wise stability, but decreases (Dutch) roll damping.

Take a look at See How It Flies, 9.1 and 10.6.1 for more details about dihedral and slip-roll couple.

Rgrds,