Pros - Wearing a cycling jersey is a big upgrade from a cotton t-shirt. It will keep you dry and cool you down when you put down the zipper. It usually has 3 pouches at the back to hold whatever you would need during a ride (spare tubes, tire levers, pumps/CO2 cannisters etc..) Which means you wont need your saddlebag.

Makes you look ...cool.

Cons - None that I know of.