No one wants rusty rebar or mesh.
At it's least harmful rust can mean stains on your concrete and at worst it can cause structural failure.
Mix designs meant to compensate for freeze/thaw conditions also increase potential for corrosion through oxidation. The same qualities of your mix meant to give water a place to expand in your concrete also allow that water to enter more easily. The presence of water and oxygen in the alkaline environment of cement are a recipe for reinforcement corrosion.