Coupon codes are useful for promos and such. Not every customer pays the same amount, thats how the world works.
The article is from the perspective of a customer without a coupon. In that case, the presence of the coupon code field actually makes you less likely to buy or convert. This article isn;t blaming the methodology. It is saying the status-quo presentation is poor and needs improvement.
On one of our products http://pressbackup.com we have a coupon code, if customers can't find one, and they email me, I will generate one just for them. Those customers stick around for a LONG time.
I love that. In start up land people always seem to ignore things that have been done successfully for years thinking that certainly there is something wrong and legacy business must be making huge mistakes and throwing out their money on things just because "it's the way it's always been done".
The problem of course is understanding which one of those things are wrong and which actually work (to which you really don't need studies) to prove.