The merchant may be very strongly incentivized to use Bitcoin because it cuts payment fraud to zero. This saves the merchant the direct cost of payment fraud, which averages 1-2% of revenue in the US (therefore up to 50% of profit margin in some industries). Note that 1-2% is an average -- some industries have higher rates of payment fraud, and they will therefore have greater savings.
As Bitcoin payments require no fraud prevention, they also save the merchant the cost of lost business from customers deterred by cumbersome fraud prevention methods, and losses due to legitimate customers who are rejected when fraud prevention gives a false positive.
Therefore, merchants will have the option of reducing prices for items purchased with Bitcoin.
This leads to a simple question: do there exist any industries where the merchant's savings could be big enough that they could reduce prices sufficiently to encourage a significant number of customers to pay with Bitcoin, while still earning a higher margin than they would from other payment methods? I think there probably are, but we're a long way from seeing this question answered by the market. Relatively few merchants accept Bitcoin, and many of them offer no special discounts.
Relevant news: http://gigaom.com/2014/04/02/paystand-takes-on-paypal-with-a...