I'm not sure what you mean that use of a payment processor defeats the purpose. The purpose is to increase sales and profits. If passing their bitcoins instantly through Coinbase for transfer to their bank account still results in enough more money to make the effort worthwhile, they would do it. I've made the point in other posts here that merchants might increase the feasibility of bitcoins by stipulating that there are absolutely no returns on bitcoin sales -- all sales final, and it would say so on the receipt. Also, bitcoin pricing could be highly flexible -- when the BTC price is rising, prices for BTC sales could be lower than the cash/credit-card price. When the BTC price is falling, such prices would be higher, maybe a lot higher -- ruthless pawn-shop rates.
What I was trying to say is that Bitcoin was originally meant, in part, to eliminate third party payment processors and the fees associated with them. Using a payment processor would be no different than accepting credit card transactions, in essence, with the need to trust a third party.
But yeah, if it brings business in, go for it. It's true that it's more viable now than ever, given the fact that you could instantly convert Bitcoins into fiat using payment processors. Consider this, though: if Bitcoin wasn't volatile, you wouldn't need to pay for their services in the first place, and you won't need to adjust prices and margins every time Bitcoin goes bananas.