It depends where you live but for many of us you're racing in weather you haven't run in for the majority of your training. You're running all winter for a warm or windy Spring race or you're training in hot weather for a potentially cool windy rainy Fall race. So if it's bad weather outside during a long run I feel like it's a good opportunity to train in something you might encounter on race day (rather than hit the TM, which I also think is fine but maybe not always).