I mixing curve can be helpful because coupling can change with increased throw. Ever do a nice KE pass at speed and everything is fine, no coupling ('cause you mixed it out like Mike said) and then try a HAKE pass with the same plane and the airplane roll and pitches like crazy? That's because the greatly increased rudder input required for HAKE has caused a different coupling dynamic. So by using an exponetial, if you will, mix we can increase the ratio or correction to rudder input as throw is increased, such as when using 3D rates.