Why do they all wear TdF leader jerseys every time they ride? it would be as if every time I went to play a pickup football game I told my buddies: "Wait up a sec - I have to put on my imitation Superbowl ring first!"
I am primarily a recreational, fitness, greenway kind of rider, but I try to wear brightly colored shirts, especially bright yellow, when I will be around cars. Visibility matters. That may not be the primary reason for some, but it is a valid one.
You can tell I'm not a wannabe pro because I call them "shirts." :)