I wear cycling specific clothing while riding. If I don't want it to fit too tightly, I'll get a bigger jersey, but I like having the pockets in the back for carrying "stuff". I also like the front zipper that allows me to unzip the front a bit and catch more air on hot days, letting the sweat wick out of my gorilla-like hair. I was at the Pearl Izumi store today, and their running shirts have pockets on the back just like cycling shirts.
Another factor is the material. After wearing Coolmax socks at work, I'll never go back to anything cotton on my feet. I now associate cotton socks with blisters and sore spots.