The myth of the "one killer message" still runs deep across a lot of business advice, especially online. And if you sells something that would be considered an impulse purchase, then it might be true that you could create one emotive message that would compel people to buy on a whim.
However, most of the businesses I work with do not sell impulse buys. In fact, more often than not, they sell things at the other end of the buying spectrum: "considered purchases". If your customers will take their time to think about their decision before they buy then you sell a considered purchase. And your customers will therefore need a number of different messages to convince them to buy.
That's not to say that marketing messages are no important - on the contrary! Creating and honing marketing messages is always a valuable strategic marketing exercise. That's why I've picked out a few of my favourite blogs on the subject from my archive and from the Watertight Marketing blog.