In running races, I wouldn't want to wear the event's shirt because it would make me impossible to find in the crowd for my husband when he tries to take a picture of me. I also don't race in clothes I don't "know" too well. hehe

For cycling... jerseys have been less critical for me than running shirts, so I don't think the last point would apply. And I do agree it's nice to see ribbons of cyclists all wearing similar shirts...