I work with a retailer who displays a coupon on every product page and it does cut down a lot of the normal brand+coupon searches - though there are a hard core who'll always look (unsuccesfully) for a better coupon off-site.
Sort of like how in Safeway every single bottle of wine is "on-sale". I know it works on many (most?) people but if something is always on sale isn't just for sale?