From a document on the exp-platform at Microsoft (download here).

Most of the changes in the upgrade were positive, but the coupon code was the critical one: people started to think twice about whether they were paying too much because there are discount coupons out there that they do not have. By removing the discount code from the new version (B), conversion-rate increased 6.5% relative to the old version (A).

Coupon code fields often make me think I’m not getting the best deal I could be when I encounter them as part of the check-out process on a web site. (I hate it when someone in a retail business asks me, "Do you have any coupons today" and I answer no — as I’m sure that I must be one of the 3 lame customers of the day who do not have a coupon ….").