Bike porn

A cycling jersey is a specialized [URL="http://en.wikipedia.org/wiki/Jersey_%28clothing%29"][/URL]jersey (shirt) for cycling. While the vast majority of cyclists worldwide wear conventional clothing while riding, a jersey offers certain advantages for the sport-oriented cyclist.
A **cycling jersey **is cut long in the back to accommodate the bent-over position used in sport-oriented cycling.
Any pockets on the jersey are placed on the back panel as front pockets would tend to spill. The zipper is often made very long so that the cyclist can open up the jersey to allow for ventilation.
The cycling jerseyc is usually worn with a tight fit in order to reduce air resistance.

Just ripped from wiki.

I would say wearing a jersey it should be tight, unless its form over function. If the latter, any pro gear away from that situation looks lame/scruffy.
For instance football sock should be pulled up.

Being a bit nitpicky here but 'technically' it kinda should :/

Basically that wiki rip is a bit mince.

Sport-oriented is a bit misleading here. Over 99% of audax riders wear cycling jerseys and describing most of them as "sport-oriented" is just plain wrong.

Pockets on cycling jerseys are mainly at the back not because of potential spillage but simply out of comfort.

Again the air resistance part is mostly misleading. The majority of cyclists that wear cycling jerseys aren't really getting any significant aerodynamic advantage out of them. If we were going for that then the market would have already moved over to the multiple fabric models used by some of the pro-teams.
It also ignores the whole area of MTB type cycling jerseys which are typically a looser fit than road cycling jerseys designed around the need for a greater amount of movement and greater protection in the event of a crash.

Not convinced about the zipper argument either, I reckon it just makes the thing easier to take off. If you really wanted to ventilate that much, you'd get ones that unzip all the way so as to reduce the parachute effect.