I disagree. Coupon code forms make some customers second guess themselves at checkout. The fewer steps required to purchase something result in higher completed orders. See Amazon one-click
I disagree with the main part of your sentiment. I think they're already there after they've filled in billing/shipping info, so seeing the blank coupon code field is actually not a deterrent at that point. It's just as easy to skip that field and press the button for Checkout as it is when that field isn't there. The coupon field just adds the possibility of getting a better deal with some Google searches.
So you have a customer that's just about to buy your product, and you're sending them away from your site? I mean, as a consumer I like discounts, but as a programmer I really want my customers to just click checkout at that point.
Speaking personally, I don't think the existence of a coupon field ever makes me abandon a purchase entirely. However, it does affect how I feel about checking out: if I find a coupon code and save some amount of money, I feel good about it; if I can't find one that works, I feel like I'm "missing out" and paying more than I have to.
