Last time I checked, there was no discount for using bitcoin. For me personally, there was no incentive to use bitcoin. As a bitcoin enthusiast, however, I always check Overstock and Newegg first when making a purchase...and I use $US. Why should I spend BTC which I value when I have USD to dump on them? Why should I spend the exchange fees to purchase BTC and send it to them?
The incentive shouldn't be to get a discount but to support both bitcoin and merchants. Why spend fiat? Don't buy from exchanges if you don't want to pay any fees.