Antiglare is viewed as a "Pro" type option. Photographers are not going to really do serious work on a 13 inch screen that is low resolution. The AG option is to provide the user with more accurate colour reproduction on the screen so it more closely matches printed images on paper.
The 13 is a consumer level computer, the decision to tag it with the Pro title on the end of Macbook is a marketing ploy to make the average consumer or first time Apple customer feel / think they are getting a Pro level machine.