In my opinion, runners make a comparable impact on soft ground as bikes so the same rules should apply. It's easier for runners to go around problem areas too, which leads to trail widening.
Red condition days are a good time to grab a Pulaski tool and make drains where water is pooling on trails and cutting out roots.... You get outside, get a upper body workout, and the added bonus of knowing the the trail will be more fun to ride and dry out quicker when the weather improves. (As long as you can move around without doing damage.)