spocla,
Dutch Roll is a mode of motion rather than a stability...
Sorry, I would have said that the anhedral increases the stability of the Dutch Roll mode, instead of saying that it increased Dutch Roll stability, if I'd known we were being pedantic.
I still submit that the purpose of anhedral is to prevent unstable Dutch Roll - the other solution is to increase fin volume for greater directional stability but at the cost of weight and drag, or to reduce sweep (but that is set by mach requirements).