It might be useful for merchant only in the case BTC price will go up and this will create an extra gain.
Merchants have everything to gain by accepting it while giving discounts as there's already a normal markup on most items. So, cut it enough for people to want to trade you their Bitcoin for products and the more this happens and these merchants keep even a small amount of their Bitcoin sales in house as time goes by, there's no way the Bitcoin doesn't appreciate. People are suckers for deals and most women have value savers tickets on their key chains for a reason.
Actually price fluctuations of BTC and high expectations of BTC holders makes BTC more like speculative asset, but not the currency. Today, to pay bitcoins for organic wine would be the same as to pay for your drink with the Microsoft shares in the 80's. So, I would not expect that merchant's discounts will "close the loop".